Below is a reconstruction of a rising-pin crossbow made by one of the society members.  This is the sort of crossbow that the Norman crossbowmen would have used at Hastings and any battle or siege of the period that Conquest covers.  You can see the step into which the string is nocked, and there is a leaver on the underside which pushes the rising-pin upwards thus releasing the string from captivity to release the bolt.  The great advantage of crossbows was the limited mount of training that a crossbow man needed.  Roughly a mornings practice makes you fairly proficient with its use, whereas an archer would take many months or years to develop a similar skill level.  An other advantage is that you can hold it loaded, where that is impossible with a bow, thus in a siege situation you can wait for the enemy to show himself and shoot instantaneously without the delay of drawing the bow.  However a good archer can shoot approximately 12/15 aimed shots in a minute, 2 to 3 is typical for the crossbow. 

Below are a variety of crossbow heads.  The two broad-heads (on the left) are for hunting, the larger being for deer, but both would inflict a fairly major wound on a non-armoured man.  The two other heads are bodkins, pointed heads which are designed to punch through shields and chain mail.  This they could do with considerable ease, much to the frustration of the knights of the period.  

As time progressed the crossbow would evolve considerably with the introduction of composite prods, later superseded by metal prods, as well as the locking wheel-nut which replaced the rising-pin and which improved the smoothness of the release.  However the super powerful crossbows of the later periods needed complicated windlass systems to load which required a lot of time to setup and use and thus meant that crossbowmen would be lucky to get off more than two shots in three minutes.  The introduction of handguns quickly brought about the demise of the crossbow from the battlefield as nothing could match their devastating firepower and uncomplicated loading method.