Dungeon is one of the strongest and my favorite 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. 2 good archers. The Eyes have 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 we shouldn't forget about the Mana Vortex and Summoning Portal. They are useful. An artifact shop and 5 levels of magic. Of course, it's hard in terms of resources, but who has it easy these days?