The crossbow is, at base, nothing more than a bow turned sideways and attached to a stock - in theory, a pretty minor variation but in practice a completely different weapon.
For a start, the stave of the bow is usually radically shortened - some primitive crossbows retain quite a long stave, but in general it is so impractical to wander about with several feet of wood sticking out each side of you that this is normally the first modification to be made. To shorten the stave and retain the same range and hitting power the bowyer must increase the draw strength of the bow - in a self-bow this would rapidly make it unusable, but in a crossbow the user can separate the actions of drawing (Often called 'spanning' in a crossbow), loading, aiming and loosing and so a heavier draw remains viable.
Once this principle had been established there was an obvious temptation to increase the draw strength further and so crossbows progressed from those that could be spanned by hand to those that required the user to hook his foot through a stirrup and use his whole body to span, those that required a crowbar like lever to span and those that abandoned all pretense and demanded the use of a mechanical ratchet device to cock. By this stage the bow could often be made of spring-steel and be alarmingly powerful and some of the heavier crossbows - known as arbalests - had their spanning winches built in.
As previously mentioned, the weight of the draw required a separation of the firing actions and the crossbow was designed accordingly - the stock included a groove for the ammunition, a hook to hold the bowstring back once drawn and a trigger to release it. Eventually sights were added to aid the aiming process. The ability to hold the bow ready to fire without fatiguing the bowman was quickly noted as an advantage.
The ammunition itself also differed from that fired from a self-bow - some primitive versions fired a round almost identical to an arrow but the bolt quickly evolved - a shorter, more robust and less refined round to better survive the more violent firing that it received. They also tended to be heavier, the better to transmit energy from the bow.
Some crossbows - also known as stone bows or prods were modified to fire stones or clay pellets but these were normally used for hunting wildfowl rather than warfare.
To understand the significance of the crossbow in warfare it is worth remembering that this was the first missile weapon in which technology began to replace skill - and therefore, in many ways, the forerunner of the firearm. An archer requires long years of practice to develop the upper-body strength and muscle memory necessary to shoot effectively, whilst a crossbowman can learn the rudiments of the task and be combat effective in a few weeks. In addition, whilst a crossbow loses out significantly1 to a self-bow in rate of fire and will normally be more expensive to manufacture it will also tend to be more powerful and less picky about its ammunition. The crossbowman faced with archers often compensated for his lesser rate of fire with a pavaise which could afford him cover whilst he re-loaded … without that, as for example at the battle of Agincourt, he tended to come off worse.
Thus, the crossbow appears to be part of the process that took warfare away from the control of the warrior and gave it into the hands of the soldier - a man did not have to spend his life training for war if a few weeks at the butts could give him the power to shoot down a more skilled man at a distance - and this leveling effect made the crossbow deeply unpopular amongst the knightly classes of early medieval Europe, even to the extent of procuring a papal ban on its use against Christians. Practicality eventually triumphed over paranoia however - and there were plenty of non-Christians who required killing in any case: so much so that the Saracens came to call it the Frankish Bow … and adopted it themselves not long after.
In comparison to a firearm the crossbow was easier to use that early cannon lock weapons, more accurate and less dangerous to the user and co-existed with matchlock weapons as well for a long time until they could be depended upon to match a crossbow for price, reliability, accuracy and rate of fire, and even then they lingered after that in places where gunpowder was rare or expensive (and even, perhaps especially, amongst men detailed off to guard the gunpowder supply).
The earliest recorded crossbow is probably the classical Greek gastraphes ("belly bow") - although some African weapons may actually pre-date these. Other classical users include the Picts and more are suspected and although the weapon is most commonly associated with medieval Europe it actually appears in a variety of places worldwide, including China where a repeating crossbow the Chu-Ko-Nu was developed.
Besides the arbalest heavier versions of the crossbow include the oxybeles and the ballista, of which the latter is traditionally used as a synonym for "artillery grade crossbow". The polybolos was an ancient Greek weapon that was effectively a repeating ballista.
Game and Story Use
- Few RPGs accurately reflect how hard it is to use a self-bow, for those that do the use of the crossbow should be a dividing line between the weapons expert and those that do something else.
- This also makes them a fairly good "home defence" weapon for civilians and the sort of thing the town guard is likely to have in the armoury.
- The crossbow was also a fairly common hunting weapon - knights who wouldn't dream of using one on the battlefield would think nothing of using one to hunt the local wildlife.
- Bow vs. Crossbow could make useful cultural flavour for your campaign - as in the real life middle ages when the Anglo-Welsh were masters of the longbow whilst most continental armies used the crossbow for their battlefield needs.