Crossbowmen are the elite missile troops in Stronghold 2. They are probably the most powerful and effective ranged unit in the game, however this comes at a price. The Crossbowman has a much smaller range than the Archer.
The Crossbowman wears Leather Armour for protection, which means that he is far slower when moving than the Archer, although he is far more protected, particularly against missiles.
It takes an Archer many shots to kill a Crossbowman- if he actually survives that long- while a Crossbowman can kill an Archer in a single shot due to the Archer's lack of protection.
Crossbowmen are slow to reload, but far more accurate and reliable as missile troops. You can recruit Crossbowmen from the rank of Knight Errant, and you will also need a Crossbow to recruit Crossbowmen. As well as a Crossbow, you will also need Leather Armour, which is why the Crossbowman has such good protection against missiles.
Crossbows are also produced through the Fletcher's Workshop, which becomes available at the rank of Knight, and you can immediately start producing Crossbows from that point onwards. Note also that by default the Fletcher Workshops all produce Bows for Archers unless you specifically toggle the production to Crossbows. To do this, simply left click on the Fletcher's Workshop and select the Crossbow icon.
You will also need a single recruit from the campfire before you can recruit a Crossbowman, as well as Honour. Honour can be produced by the means shown in this article.
Tip: Since Crossbowmen are slower than Archers and have a shorter range, they are best placed protecting walls and towers which will allow them to fire further. It also protects them as they are a slower moving unit that cannot move out of the way as quickly as Archers can. Crossbowmen can easily dispose of troublesome units such as Knights and in a group they are a lot stronger as well, doing far more damage.