All castles in Heroes 3.

The undead are invincible, especially Galtran. He can only be defeated at the very beginning or relatively early in the game... otherwise, as mentioned above, 3,000 skeletons are equivalent to a victory. Galtran always gets Air and Earth, so using two spells – either Haste or Slow – will help defeat him. Blindness doesn't work on skeletons, so you can't cast a lot of spells and then shoot them down... the skeletons will simply approach (either they are hastened, or you are slowed).