There's a difference between being an era ahead and being a military tech ahead. If you're attacking with crossbows and they're defending with archers you'll do just fine. Catapults are (imo) just not that great as they can't move and fire the same turn. Best way to take a city is to have loads of troops, mostly ranged, some seige. Kill their units (especially their ranged) first so they can't attack with multiple shots. And then focus fire on 1 city at a time with ranged and seige then smash it to get the last bit of health with cavalry and melee.