In theory, any race in a city is very strong. If an Orc has battle cries, it's problematic to defeat him. The same can be said about the Elves – try attacking their castle. One spell, and the catapult is gone. Second, there are no archers, etc. Or mages with three archers and probably a hero who uses Chaos magic. Or Elves with their super-archers. However, for races with powerful Chaos magic, besieging a castle is not as difficult as it is for warriors.