The Castle is built around defense. One building strengthens walls and towers. Another allows deploying an additional unit on the battlefield (as far as I understand, but I couldn't verify it since no one attacked). Plus, in the Castle there are buildings that increase troop growth for level 4-5, which also seems decent. The castle's development tree is quite interesting; a smithy is needed for money (like the mages guild at the Academy), so perhaps War Machines is one of their primary skills....
I didn't have time to look at all the heroes, but so far I was pleased with the hero with spear-throwers; they somewhat reminded me of the assassins in the Dungeon.
Anyway, if anyone has already played, let me know...