Siege weapons!

I have a habit of stealing settlements from the enemy and then letting them break their armies on assault. A ballista or catapult set facing the road down to the gate causes a healthy amount of havoc, and can occasionally engage siege towers too. As for bridge defences - it probably amounts to cheating, but there's something amusing about having a single catapult unit rack up 500 odd kills. Armoured sergeants and catapults are the basis of my defense of Outremer from those nasty Turks/Egyptians/Mongols.