Dungeon is one of the strongest and most beloved castles. It has quite strong heroes (Malekith, Jedite). Troglodytes are not the worst option. Improved harpies are simply delightful at the beginning of the game. Two good archers. The Eye has no penalty. Medusas petrify. Minotaurs are great with their morale. Scorpions are also good. Chaos Devils have always been and will always be the best.
And don't forget about the mana vortex and summoning portal. These are useful things. An artifact shop and 5 levels of magic. Of course, it's difficult in terms of resources, but who has it easy these days?