Does anyone know exactly what fort level does? I'm wondering if it is worth boosting or whether I should spend my money on troop increasing buildings.
It slows down enemy sieges.
It make siege last longer and act has a force multiplier for the defending force should the defender decide to assault. No idea of the exact calculation but just look how much longer it take to siege and how many casualty you take assaulting a castle which has a base 3 fort level over church land which have a base 0. So basically it let you sit back and wait for the situation to upgrade itself (like getting money to recruit mercenary or levy to come back to make other assault) or keep enemy busy by doing nothing (also wear them down since longer siege mean angrier vassal =p)
But I admit early in game the levy and economic increase building are much more important.