All castles in Heroes 3.

I don't really play on difficulty 200... but on 200, even the mages are in a tough spot...

Nah... I wouldn't say that... the difficulty gradually becomes manageable; for me, on Master difficulty with the Gremlins, I only need gold for building. And the ranged unit at level 1 and the fast-flying unit at level 2 (the Gargoyle) are excellent for the start of the game, believe me!!!
P.S. I only play on difficulty 200, but it took me a long time to get to this level ;)