Attackers don't lose men while sieging. Defenders do. This is kind of weird, as MTW-VI insisted on the "realism" new feature that forced gamers to attack castles because of the attrition rate.

IMHO, some attrition should be experienced by attackers in any situation, even when marching, somewhat like the Europa Universalis system. Anyway, to have the defenders die in the city and the besiegers thrive outside is unrealistic at least.