My brother asked me to write this for him, as he was planning to run a campaign in a game world resembling England in 1200 A.D.
In Britain, the "Early Medieval" period lasted from the sacking of Rome in 410A.D., until the arrival of William the Conqueror in 1066. After this, came the "Medieval" period, which lasted until the Battle of Bosworth in 1485, after which there came the "Tudor" period. This source material will be useful for the medieval period, particularly about the year 1200.
1200 is after the start of the crusades and the contact with the Arabs, but before the Tartars/Mongols fought European armies. In Britain, the land was one with many rival barons, who had serfs to repress and control, the King to vie with for power (ending with some success: the Magna Carta 1215), and each other to beat up. Don't forget too, that Britain and what is today (most of) France were under the same rule.
BUILDINGS and ESTATES
Most castles by 1200 would be of stone, mostly with square towers, though the round tower was just being phased in. Castles as we know them were introduced by the Normans, and some of the older or poorer ones would still be of the motte and bailey kind, or conversions from them. Most domestic houses for the poor would be of wood and thatch, though most manor houses would by now be of stone, and have more than one floor, which means that central fire places are on the way out - fires are in fireplaces on one wall of two-story buildings. The latest thing in 1200 was the tiled floor, which the most with-it rich people would have. The lords had stewards under them to run things, and under them were the bailiffs who would occupy the manor houses temporarily in the absence of the stewards and lords.
Peasants had little plots around their houses with vegetable patches. Fields were still on the Saxon open field system (fields with no fences around them, one furlong long (one plough furrow, or 220 yards, or an eighth of a mile), and of a few standard widths, four and eight yards being the most common, with a reverse S shape in plan view. The overseer of these fields was the "reeve", often elected by the peasants, to settle disputes.
The manorial estate was the basic unit of the feudal system. Part of the estate was the "demesne" of the lord (who might have a few estates). The rest was farmed by freemen who paid rent, and by "villeins" who were not allowed to leave, and who had to work two or three days a week for the lord, as well as providing produce for the lord at certain busy times of the year. Rents were unpopular, but less popular still were the laws enforcing everyone to have their wheat ground to flour at the lords' mills. Millers were commonly portrayed as dishonest baddies. Mills were often water powered (Domesday book (1086) lists 5,624 in England), muscle powered, or very occasionally, wind powered - the first windmills appearing late 12th century (advanced ones could be rotated to face the wind, but came later - 15th C). There would be one for every fifty households or so (remember, a household is big, and more than one building - no nuclear families like today).
A man might be made a knight and granted a fief, which was an area of land which he then used to support him and raise him enough money to equip himself as a knight. In return for this, he became the vassal of the man who granted him the land, and would fight for him.
Towns were not very big. Only about five in England are thought to have had over 2,000 houses. Population examples: Newcastle 10,000, York 15,000. The estimates for the total population keep changing. Two million is one I heard recently. Roman Britain had about seven million.
Rights to hunt and fish were very jealously guarded, and commonly, it seems, abused. The right to hold market was granted by the king, who charged for this, and this was a quick way for him to raise money. The head of the local merchants' guild was the "alderman".
Cooking was still commonly done in cauldrons, but this does not mean that people ate huge stews all the time. With skill, a cauldron can be packed with many containers with different parts of the meal in them, and which can be fished out at different times, while things on top steam. Pottery was often "green glaze" which is green, and has a glass glaze making it totally waterproof. Spices from the east, like ginger, cinnamon, sugar, cloves, as well as some fruits, like dates, also rice and silk, were first becoming well known in Britain at around this period.
Torches, as beloved of film makers, were not the norm. Torches are a fire risk, and do not last very long. Oil lamps, often built into a wall (a "crennet") were simple, shed a more constant light, and were cheaper and lasted longer. Portable simple open topped lamps were "cressets". My extensive experience of live action role playing at night tells me that unless conditions are unusually dark, it is much better not to use light anyway, since you only blind yourself with it most of the time.
Looms were still vertical. Who says a section has to be long?
Mechanical clocks come later. Water clocks and sundials are more in period. Monasteries would have these, to regulate the times for prayer, and people could pace their day by that. The day was still regulated by the Roman method: daylight being divided as night into twelve parts, therefore a daylight hour in summer was longer than a winter hour.
The British were the people in Europe who had perhaps the most respect for infantry. The French were particularly noted for the utter contempt for footmen, whom they regarded and used as expendable battle fodder. Some small infantry armies of gallant Brits consistently stuffed the French for some considerable time, notably at Crecy (1346), Agincourt and Poitiers (1356). Still, though, in this period, the proper place for a noble knight was in the saddle. As time went on, knights were more and more prepared to fight on foot.
Chainmail was the armour of the period. Some people harp on about this term's being wrong, saying that "mail" is correct. I argue that "chainmail" is perfectly acceptable, since it is unambiguous. The term "mail" was used to refer to a knight's armour, and since this was near enough always chainmail, the distinction is lost. Anyway, mail or chainmail was pretty uniform. The fact that chainmail lasted from the iron age (possibly even earlier) up the twentieth century shows that it must be good stuff, since everything else came and went.
Mail varies a bit in quality. Not all of it was riveted, and some was double riveted. Some was very fine, some chunky, but you may not wish to distinguish all that much. Some of the very finest mail was of limited practical use, but looked great. Most of the mail was pretty much of a muchness, though. Dark age warriors tended to wear a byrnie, like T shirt, of mail, and this extended to a hauberk (like a dressing gown which covered down to the knees and had half or sometimes full length sleeves) which was common by Hastings, and this in turn extended again to cover the whole body, including the feet, head, and hands. The mittens which covered the hands in battle were commonly part of the sleeve, and a hole in the palm allowed a grip on the sword, and allowed the knight to pop his hand through when not in battle, to manipulate things easily, with the mitten hanging from his wrist. So far as I can tell, the mail often covered the sole of the foot, which would I imagine afford a good grip on all surfaces except stone. The head was often protected by a coif, which is like a balaclava of mail, which was always separate from the hauberk. So far then, you have three degrees of coverage: byrnie, hauberk and full coat, which your rules may like to distinguish.
Note that mail was almost always padded. A coif with no padding under it would do little good at all. Think about it. Arming caps were often made from coiled rope, and often involved a lot of felt padding. A coat called, among other names, gambeson and akerton, was worn under mail, and was heavily padded, materials including felt, leather, cloths of various sorts, wool, and something which one sometimes reads described as "kapok" but I wonder if they can mean the same stuff. Mail is quiet, comfortable, can be made to fit someone without the need for accurate fitting, so can be stolen and worn by anyone of similar size. Mail without padding should be a lot less protective, but can also be worn under ordinary clothes without being detected. Mail acts like a heat sink, keeping a man cool in summer and cold in winter. Padding, in summer especially, was often worn over the mail to deal with the heat problem. If it was not, then in very hot weather, a linen tabard or surcoat was put over the top to stop the sun from heating it up too much.
Helmets were changing in 1200. They were worn as well as coifs. The first of the new conical ones designed to make blows glance off were coming in. Older style close helms had flatter tops. Noseguards were going out of favour by 1200. One reason for this may be that they afforded an enemy a good handle by which to grab your head in close combat (King Stephen was captured this way in 1141). Another might be fashion, which one should not ignore, and another is that they do a lot to disguise a face, and heraldry was in its infancy, and the rest of a knight's attire was a uniform mail. The new problem, though, was that blows landed on the shoulders instead, which then got broken. Ailettes were introduced, which were plates, often square and flat, worn on the shoulders, to allow blows which came off the head, to continue on their way. These often had heraldic devices painted on them.
Another kind of helmet around in this period, taking over from the flat topped kind, was the "great helm". Some were barrel shaped, other faceted. Holes called "breaths" were cut in the front, and the eyes looked through big slits. This helmet was carried most of the time on a chain on the saddle, and only worn when charging in to battle. In close melee with many opponents, it would be ditched to allow better vision and breathing. Under it would be worn (typically) a padded coif.
The first plate armour worn elsewhere in this period protects the knees (poleyns) or the lower leg (greaves) which are especially vulnerable. The next part to be plate armoured was the elbow (couters). Knights aren't wearing full plate until c.1360 A.D. Brigandine is later, too - 14th C.
The lesser footmen might wear an iron skull cap, quilted leather jacket, and carry a buckler - a small iron shield. The buckler especially allowed greater manoeuvrability on the battlefield. Veteran footmen would be likely to have looted armour from earlier campaigns, but a knight's armour would all fit him well and match.
Shields are excellent things, and until this period all armies used them extensively, and they tended to be large. A large shield is encumbering, but good for fighting in groups, and good against missiles. Around 1200, fighting is less formal than in ancient times, and people preferred shields that were a light (mostly wood and leather), and for use in groups as well as individually, and so preferred a medium shield that was the traditional pointy at the bottom shape which seems to be particularly suited to horsemen (a "heater"). The Norman kite shield is still around, but not standard. Part of the reason for the lack of large shields for this period, is that armour is improving and getting heavy, and more reliance is placed on it for protection from missiles, and so the shield is less important.
In 1200, the spear is still the main group foot weapon. The Swiss, and to some extent the Scots, are using pikes. As armour improved and shields became even less important, two handed cut and thrust weapons such as billhooks, halberds and all that lot, gained favour, and shields were discarded. Around 1200, though, the spear is still popular with massed footmen. Short spears are about the height of the wielder, long spear are about eight foot, lances about nine (often cut down during battles). On foot, a man holding a shield in one hand can fight with a spear up to eight feet in length in the other. Beyond 8', it is impossible to fence with a spear one handed, and one either has to use both hands, in which case an 8' (shorter) spear would still be more wieldy, or one must be in a very big and well drilled formation, and just walk forward with it, presenting a fence of points to the enemy, since one cannot rely on parrying and the like to ward off blows. Pikes (up to c.18'!) are especially good at keeping cavalry at a distance, but spears are pretty good too. Short spears are more wieldy in individual combat than long spears, which are relatively awkward. Your rules, therefore, might care to make distinctions between shorter more wieldy spears, longer spears, and pikes. Only a one-armed man would use a spear one-handed if he had no shield. A spear used two-handed is much more effective.
Swords seem to become more common and yet less used. Footmen start carrying them fairly commonly. They were always the badge of a knight, however, and all knights had swords, which tended to have the Christian cross like shape. Cutting swords had more and more trouble penetrating the increasingly heavy armour, however. They got narrower, thicker, and more pointed, to make them more effective at thrusting through mail. They were diamond shaped in cross section of the blade. The falchion, an eastern style of sword, came in. It was wide, heavy and single edged, a cleaver for smiting opponents with. You could define stats. for two types: piercing and slashing.
Given that swords were less effective, axes and maces increased in popularity for use by knights in close combat with other knights. These were short and brutal if one handed, and a two handed axe existed which knights might use in this period (remember that felling axes designed for use against trees are very unwieldy weapons). In 1200, however, swords still hold sway. Give them stats. which make them more wieldy than axes and maces, better at parrying, and doing less damage. Morning stars were later, and favoured by Germans and Swiss.
Near enough everyone would have a knife or dagger. There is no substitute for one of these in very close combat. They are very quick, and can find the chinks in armour better than anything else.
Foot soldiers were often hurriedly recruited, and would have a variety of implements for use in melee: flails, sickles, hayforks, fencing hammers and clubs. Fencing hammers were very effective at stunning a fallen knight - bash him on the head with a big wooden mallet and he'll be stunned no matter how good his helmet is, then get in and stab him with your knife. These mallets are also good for making fortifications, such as banging in stakes.
Greek fire was occasionally used, though I know of no reference to its being used in Britain. This could be thrown, or blown by bellows through tubes. Water would not extinguish it, but supposedly urine, sand and vinegar would. It is used in sieges and sea battles mostly. Gunpowder was very rare. One Christian monk listed its ingredients in 1242 A.D. It didn't change things that much until quite a bit later. In 1449 A.D., castles were falling to cannons in France at the rate of five a month. The first reference I know of gunpowder weapons being used in European open battle is 1346. The first guns were more frightening than deadly.
The two main missile weapons are the long bow and the crossbow. Composite bows would be known from the East, and self bows too, but these were the two main bows in use in battles in Britain and Europe. The French favoured the crossbow and lost, while the British favoured the longbow and won. Eventually, even the French had to admit that the longbow was a better weapon in open battle. Long bows were usually made of ash or yew. Yew was a sacred wood, often grown in churchyards. Poisonous to sheep, I'm told. There were laws against uprooting yew trees, which are slow growing, and seldom straight. You can only get about six bows from one tree. Yew loses its strength quickly and yew bows didn't last long, so many spares had to be carried in carts for battles. Ash lasts longer.
Longbows have good range, and in the hands of an expert, good accuracy. The rate of shooting was phenomenal. An archer could shoot an arrow while the previous two were still in the air. There are claims for as many as twenty shots per minute. Ten shots a minute is certainly possible, though with careful aim at an individual target this is more like six per minute which is still pretty good. Archers could form up in deeper formations, shooting safely over the heads of their fellows. They could change their bowstrings mid battle, for dry ones kept in pouches, while a wet string on a crossbow required some time in a workshop to change. Long bow arrows were a good yard long, and fairly heavy, such that just the weight of one falling from the sky at maximum range was enough to wound an armoured man. Even when arrows do not pierce, they break up formations of troops, and they make it physically difficult to advance against their blows and over shafts stuck in the ground. At close range, shot on a flat trajectory, they could pierce any armour (though armour still helped a fair bit - not every arrow would hit flat on and penetrate, and those that did would have some of the kick taken out of them).
Bows varied in power, and this varied with the man. Strong men would get bows suited to them, as would weaker men. Assuming that a man is using his ideal weight of bow, the power of it varies with his strength. Composite bow technology allows powerful bows to be made smaller and lighter, useful for horseback use, but still the strength of the man is the limiting factor. Therefore your rules should assign damage according to the strength of the archer.
Arrows were of several designs, but some distinction in rules should, I think, be made between ones designed for piercing armour, and those with wicked barbs on which would cause terrible wounds and were difficult, dangerous, and painful to remove, though far less good at piercing armour. The English did not use quivers much, and instead had sheafs of arrows tucked in their belts, which they would stick point-down into the earth to be in a convenient place for rapid shooting. The earth on the points sometimes contaminated wounds, and the French accused the English of poisoning their arrows.
Crossbows had even greater ability to penetrate, but only at short range. At long range, their lighter shorter bolts were less accurate and a lot less powerful. They were good in sieges, where speed of shooting was less important. It took less strength and less skill to shoot one properly. One could keep one cocked for a fair while, and one could take cover kneeling behind cover with one more effectively. There are several devices for loading the crossbow, but I would divide these into two kinds: those that require the bow to be loaded in a single movement, which do not work on such powerful bows, and those which allow the loader to stop and rest part way through, such as a windlass, which allow the loading of a more powerful bow.
Artillery had improved a great deal 1100 to 1200, thanks largely to the experiences of the crusades. The efficient designs of the Greeks and Romans had been forgotten, but mangonels (torsion powered - the ones with an arm with a load of twisted ropes at its base, which flies up when released and hits a high horizontal bar) and trebuchets (the huge ones with a big weight at one end of the throwing arm, which falls, lifting the stone throwing end) were now around.
That's most of what I have to say about weapons. I think that your game universe will benefit from keeping the number of weapon types down. Elephants, camels, slings, javelins, staff slings, shortswords, pavises, pila, lassos, and all the countless other weapons of the ancient and medieval worlds were rare to non existent, so why clutter your campaign with them? Better to have a feel for period and culture and leave them out. You have enough variety in what I've said already: group weapons, duelling weapons, long, short and close range weapons, knightly weapons, footmens' weapons, peasant weapons. The story should not be about the weapons, but about how they are used. Gone shall be silly studded leather, horned helmets, and double bitted axes, which are all figments of the imagination.
The AD&D system is fundamentally flawed, and I urge you in strong terms to ditch it and replace it with something half decent. In AD&D, an unarmoured man with a sword is as easy to hit in melee as an unarmoured man without a sword. This is flagrantly ridiculous, since weapons are used to defend oneself. Similarly, two 20th level knights will both find it easy to hit one another, since both are skilled at attacking. Is neither skilled at defending? Your combat system MUST take into account the skill of the defender. RuneQuest does this by having parrying and dodging (dodging is especially fun for those who like to play swash buckler types who wear nothing but a white shirt, and a fine moustache and evade hurt by lots of leaping on tables and chandelier swinging). There are other ways.
Remember, wearing lots of armour makes a man slightly easier to hit, but armour absorbs damage. Any system which suggests otherwise is flawed.
Horses then were much like cars today. That is, that most people who really wanted one could get one, but most people had just one, and not a great one at that, but instead a working practical horse. The horse equivalent of the sports car was a richer man's thing, and only rich men had stables of horses. Think of a warhorse or a mail shirt as costing the same as an ordinary man's house. That would fit the expensive car analogy. Stirrups are now pretty standard, at least for fighting. War saddles are high at the front and back, to hold the rider in place.
Some prices for a knight's equipment: horse = 12 cows; mail shirt = 12 cows; lance and shield = 2 cows; sword and scabbard = 7 cows; helmet = 6 cows; leg armour = 6 cows.
The penny was the standard monetary unit, of silver, and often cut in half or quarter for ha'pennies and farthings. Four pennies was one groat 1351 - 1662. Unfortunately, I don't know how much a cow was!
A common form of contract was the "indenture", a piece of parchment or paper on which the agreement was written twice in ink (on lines ruled in lead with a stylus), and then torn in two, between the two copies, in a deliberately ragged way, so that the two halves could later be matched as proof of the agreement.
Cathedrals were already getting pretty spectacular, Durham Cathedral, the greatest of its time, was already old.
There were many disputes, of course, between the various religious orders. A common theme was the conflict of ideas between the solitary, poor, hermit-style monks, and the rich organised monks. There were also old/Celtic and new/European conflicts, the Celtic churches having more emphasis on nature and life. Monks often ran model farms, "granges", and these were often successful. They also ran other businesses, such as kilns. The warrior orders, such as the Templars, also had rich granges. The warrior monk, as part of Christianity, was something created by the crusades - they were people who volunteered to guard what had been captured, and the early ones were ex- pilgrims. The fighting holy orders of Templars, Hospitallers and the like became rich, and later members thought more of managing their estates than of fighting heathens.
Pilgrims wore a standard set of clothes while on pilgrimage, and badges were worn by people who had done pilgrimages, which showed where they had been. Finding relics was a good thing to do. A relic could be sold for enormous amounts to churches, which competed for them avidly, and used them as tourist attractions. Hospices were set up as stopping points on pilgrimages, and become homes for the elderly and ill. There was hygiene and care, but very little knowledge of medicine. Rich people were treated at home. A lot of medical knowledge was Arabic. Lepers were treated like lepers: forbidden to live in cities, enter streams, churches, mills, taverns, bakehouses, or walk unshod, and they had to carry rattles and wear distinctive clothing to warn people.
Falconry was the sport of the nobility. Nobles would often be seen with their favourite birds, which might perch behind them at meals as a status symbol (remember, the female is called a falcon, and the male is a tercel or tiercel). Also, knights almost never travelled alone. Travelling alone was what poor people did.
Festivals were very common - up to fifty a year, and these would be mixed blessings for the poor, since they would be holidays without pay. Mummers were (occasionally banned for being a bit pagan) groups of players in gaudy dress and masks who put on short plays, typically involving a few common characters: St. George, Slasher, the Doctor etc.
Jousting and the tilt came hundreds of years later. In 1200 A.D., tournaments were mock battles and often quite bloody, in which knights sometimes became "possessed by the devil" and killed each other. The churches often opposed them. Some knights were like professional players, in it to win money and equipment from defeated foes.
I hope that this helps. A quick source book for the period. I think that there are a fair few ideas for scenarios lurking there: broken contracts, disguised lepers, leper disguises, pilgrims, relics searches/rescues/stealing/forging, disputed rights and land, rigged elections, bloody tournaments.