https://www.youtube.com/watch?v=MMoL_SBD6gw

This is a great video about medieval crossbows. One advantage they have is you can train men to use a crossbow in a mater of days. To use a longbow was estimated to be twenty years. You would however sacrifice the higher rate of fire the longbow possesses. The Crossbow by Sir Ralph Payne-Gallwey is a good source if you are inclined to like medieval weaponry.