More precisely, it's a faction; a castle is a defensive structure.
You'll say no?
Well, like, all the units in Inferno are so strong, muscular, and tough-skinned (except for the succubi), so they must be the most resilient.
and the Necropolis in Heroes 5 can't boast such speed; look at the ghouls with the ability against the living, they cleared the whole map.