Castles make them bombard at 4% per turn, so you need 25 bombardment turns to get past any one castle. They always inflict less collateral than catapults given the same promos. They are only marginally less expensive than cannons. Despite these things, they DO still get winning odds against non-protective, non-hill longbows and come with a tech that might just be necessary for counter-mounted. Even cities with 8-9 defenders will generally lose about 1-2 trebs max to kill every single one. The problem with medieval was is that it is slow, which means successful medieval wars combine war with infra development. That's not a cakewalk to get used to, but it can work...especially against disadvantaged opponents that lack iron or horse and thus can't reliably flank siege.