What do you mean, where? There's a Castle of the Elves, and there are 2 Elf camps. Where are the 1250 Assassins?
The mine guard is part of a quest. And you'll have to break through it with what's left after that (and there will be almost nothing left, because the Demon has Blind, 280 mana, and a bunch of Imps that will drain his mana, depriving my hero of it + 19 attack).
