Crossbows
From Crusaderkings
Crossbows are hand-held ranged weapons with a shorter range than bows, but greater striking power, especially later in the Middle Ages. They were the preferred missile weapon in Central Europe and the Dutch city-states. Bows are generally a better weapon in CK, but crossbows have considerably better modifiers against heavy infantry. Note also that bows will upgrade both Archer and Horse Archer troops, while crossbows will only upgrade archer troops.
Slinged Javelin (Level 1): Attack 5
Archers: No bonus.
Light Crossbow (Level 2): Attack 7
Archers: +30% against heavy infantry
Medium Crossbow (Level 3): Attack 9
Archers: +60% against heavy infantry, +30% against knights
Heavy Crossbow (Level 4): Attack 12
Archers: +100% against heavy infantry, +60% against knights
Arbalest (level 5): Attack 15
Archers: +150% against heavy infantry, +100% against knights