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Okay, I just tried that.
I conquered the turquoise Castle. It's located if you exit the main Castle and go forward along the edge of the map.
Then I returned, gathered more troops. I had all the upgraded units except for the Orcs before the Shamans.
I went and killed the Red player. He's in the Orcish Castle, to the right of yours, if you consider your Castle to be behind you.
I conquered it. Then, a magical orange portal appeared, right near the exit opposite the second Orcish Castle. I defeated the Orange player (he had a very low threat level).
I went back to my main Castle, gathered an army. I was making a lot of money, enough for the army. After a week or two, I went to the Red Castle. It's like going to the second Orcish Castle, but further.
At that moment, the Orange player wanted to attack me, specifically the Castle of the Dead. Well, I defeated him with the same Death army. My strongest units were Vampires. I conquered the Red Castle. The Red player only had one unit left. She ran out of the middle orange garrison. I defeated her too. The Red player is defeated.
Now I'm thinking about gathering armies from all the Castles and converting them to undead.