I completely forgot to mention that this post is some kind of followup to Conley’s thoughts on crossbows.
29. We prohibit under anathema that murderous art of crossbowmen and archers, which is hateful to God, to be employed against Christians and Catholics from now on.
Crossbows are an amazing weapon: if you’re not one of the few people innately good at it, bows are notoriously a pain to leant to use properly. You need good posture, aim, breathing, upper body strength, tons of practice. But as England brilliantly showed at Agincourt against France, bowmen can win a battle. If put on horses, like Partians and Mongols did, they can be even more effective, attacking the enemy while keeping at distance.
But still: lots of training. Training is expensive, in a way more expensive than mundane goods as it can’t be easily replaced. The same can be said of the warrior class of the middle ages, that in the western world went through years of training before being the armed and dangerous weapon of mass murder, plunder and destruction that all players of fantasy RPGs learn to love.
Enters the crossbow: technology offers a missile weapon that is way easier to operate than a bow, can scale in size and lethality easily without much problem for the operator, due to its construction is smaller and at the same time sturdier and more delicate than an equivalent bow. Combining all these factors, if a sufficient number of crossbows and darts can be made by skilled and specialised artisans, villains and citizen can be made crossbowmen in a short amount of time (more or less the same can be said for pikemen used in a defensive formation, as the Swiss demonstrated to the Holy Roman Empire).
The reliance on specialised craftsmen to produce tools which automatise the difficult part of the work (crossbows and war, in our case) mirrors, in a way, the construction of mechanical weaving mills by engineers, to be used by unskilled labourers to mass produce textiles at the beginning of the industrial revolution, centuries later.
Knights fear crossbows and bows because of their inherent subversiveness: armours can be punched through by arrows and bolts and, while being killed by a lowly welsh bowman that spent lots of time practicing with his bow is a terrible faux-pas, being killed by a pig herder with a dumbed down bow is a sure way not to receive R.S.V.P.s to dances, parties, wedding and tournaments. In Western Europe the social pact between worker class, warrior class and clergy was menaced by this, and Pope Innocent II made the use of said missile weapons illegal against Christians, which did not stop Agincourt to happen and crossbows to be used. Or gunpowder weapons to be developed.
McNeill in his book Pursuit of Power explains how in China an early “industrial revolution” of sorts produced enough iron (combined with an adeguate supply of craftsmen) to make the production of significant amounts of crossbows feasible. For a serie of reasons, the Chinese bureaucracy, clergy and worker classes excluded the warrior class from the social pact, as it suited them better to have masses armed with locally made crossbows than have to deal with warlords in friendly terms.
Meanwhile in Europe Balestrieri Genovesi, Genoese crossbowmen using weapons made by the balistai guild of the Most Serene Republic of Genoa, were used by the City as defence in both the army and the navy, and as mercenaries across Europe, apparently causing plenty of damage almost against everybody they were pitted against. And when captured they apparently enjoyed fingers mutilation as bowmen did.
D&D fails with crossbows. They behave the same as bows, and the heavy crossbow from BECMI is a step in the right direction but doesn’t quite cut it. Which is a lame and not at all badass if you want my opinion. d20 does much better, listing the heavy crossbow amongst simple weapons, dealing plenty of damage and being usable by classes, to reflect its ease of use.
My homebrew D&D will soon have its version of the “knight killer”: 1d10 of damage, two rounds to reload and if AC 9  or better is hit, add +4 to hit to represent armor penetration; it’s a riff on the mechanic I’d like to use for axes and maces, in turn a simplified “weapon vs armor class table” from AD&D.
But if a crossbowman with his death implement happens to be around guards from a local knight or cleric, woe to him. Knight killer crossbows are illegal almost everywhere outside the domains of Free Cities: as the only purpose of having one is to kill or harm an armoured target, and the only relevant armoured target in a fief is the local vassal, even the possession of a k.k crossbow manifests a want to plot against the life of the local lord.