I've never had high casualties going after 1 man in a fortress. Each wall you take down by 50 percent or more stops that wall from firing. So once I breach the wall I start knocking down all the other walls halfway and only send in troops once the siege equipment is out of ammo. I've never taken more than 20 losses I believe. Also I send in cavalry even though they are bigger targets they get there a lot faster and run under most of the arrows plus if most of the walls on 1 side are knocked down it wo'nt be until they are right up near the castle until they come under fire. Another thing is a castle or a fortess with a ring wall the A.I. never camps out in the inner keep. The A.I. always runs to the other side of where the siege engines are at