Originally posted by Beryl
Indeed, it was usually due to these archers that the English invaders were able to kick French butt for most of the Hundred Years War.
Though to be fair to the French, the crossbowmen employed at Crecy were Italian mercenaries (Genoese). Of course, it didn't help that the French gend'armes galloped through them...
But the fate of the Genoese crossbowmen in 1346 points to some problems for that weapon in general. First, as has been pointed out numerous times here, the crossbow had a much slower rate of fire: I've seen reenactments in which a longbow archer shoots ten arrows in the space of time it takes a windlass crossbow to shoot once. The Genoese barely had time to march into range and shoot before being driven from the field.
A second issue is trajectory. For some reason (physics is not my strong suit), a crossbow bolt flies on a flatter trajectory than an arrow from a bow. The Genoese had to march uphill at Crecy and shoot at the English on higher elevation, and reportedly their bolts all fell short. The crossbow was best used for a straight shot; this may be why crossbowmen were historically deployed to open a battle, because they couldn't effectively shoot over the heads of infantry/cavalry in front of them.
A third issue is spacing. The Genoese at Crecy apparently took up a wide front, or else the French cavalry could have gone around them rather than through them. Shooting the crossbow required the shooter to face forward with his full body, and the bow itself has considerable width. Longbows, on the other hand, require a much narrower profile from the archer, who can stand basically with his side profile. As a result, longbow archers can be packed more densely into a smaller front: in military terms, a greater concentration of force. Given this limitation, and combined with the trajectory issue above, it is not surprising the crossbows were employed most effectively in castle sieges, where they could be loaded and shot at leisure behind fortifications.
Ultimately, the crossbow's biggest advantages were ease of use and force of impact. Especially with the advent of drawing-cracks and winding-winches (the arbelest and windlass crossbows), heavy crossbows developed an effective "pull" far greater than normal human muscle power. Someone previously mentioned that crossbow technology compelled the development of plate armor. Although longbows, too, seem to have been able to penetrate mail and even some early plate armor quite effectively, when plate armor reached its peak from 1450-1550 it was tested against the crossbow. Plate suits were often called "proof armor" because of the small proving dent inflicted by a crossbow test-fired against it at close range.
Back to Middle-earth, it is difficult to know for certain why kind of historical bows influenced Tolkien's thinking. Did the Free Peoples wield stout English-style longbows, which is to say composite bows of yew and oak? Or did they simply carry oversized versions of older, single-stock bows in the Frankish or Viking style? English-style longbows did not appear in Western Europe until the 13th/14th centuries, so it is hard to say if the hyper-ancient peoples of Middle-earth would wield similar weapons. Tolkien didn't mention crossbows by name, though he did indirectly hint about "machines" used in war, and the mighty siege engines that he mentioned certainly included crossbow-like "ballistae" in the medieval style. So, when trying to make crossbows for LOTR, how good they are should depend on how good you envision the bows being. If you think the Free Peoples are wielding English-style longbows, then the crossbow in comparison should be a primitive, slow, and relatively weak weapon employed by foolish Orcs. But if you think the Middle-earth bows are themselves more primitive, then in comparison the crossbows could shoot farther and harder in exchange for a considerably slower rate of fire.
Wow, I didn't sit down to write anything this long. Sorry!
Scottomir's LOTR Game Resources:
http://www.geocities.com/scott_metz/