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I just finished playing this mission. I personally didn't have any problems defending the castle. Instead of buying the native hero, I bought a Warlock and started leveling him up in magic, so I could easily defend against all attacks with the hero's magic and the druids' magic, along with archers and ents. And I ran around with mostly dragons for almost half the map. I cleared the entire map, visited all the upgrade locations, and finished in 3 months, 4 weeks, and 6 days. I could have done it faster, but I decided to visit the magic guilds with the freed heroes to level them up. Now I have to chase Tieru... again.