Inferno tactics.

If you agree to a truce for the first 2 months, you can defeat any castle with this strategy (except for a Colony or a Conflux, as many others have). First, build a Capitol, then buy all the creatures (but not the upgraded ones). After that, gradually build the upgraded creatures (while you're building, a strong army will accumulate). If you don't like Demons and Pit Lords (level 4 and 5 creatures), you can leave them with Imps. If you've learned Armageddon, try to win with only Ifrits, and leave the rest of the creatures in the castle. The hero in the castle should have improved attack, defense, and ranged attack. The hero with the Ifrits needs to know Wisdom, Sorcery, and Fire spells – absolutely! And preferably, protection from magic.