I liked the map. It's simple, beautiful, and charming. If someone is looking for a hardcore experience, this isn't the place. I'm not sure if there were any attempts to strengthen the AI heroes, but they didn't seem particularly aggressive to me. Yellow and Green showed up once on ships. Yellow defeated Blue, which I was very happy about, and Green was stopped by my Castle, defended by a secondary hero. Shortly after, all the Castles were under my flag, and only Orange remained. Orange sits quietly, doesn't go anywhere, and therefore didn't last long. I was playing as Necromancers, and the army was assembled very quickly.
If you want to increase the AI's resistance, you should disable seafaring for it. Instead of developing a hero, the AI always prefers to send it overseas. After losing a hero, it sends another one the same way. There are almost no garrison troops left in the Castle, and the hero who sailed away can neither capture another Castle nor return to its own. Perhaps you could give Orange the freedom to move up to the gates with Phoenixes. That way, it would be very easy prey. The Castle walls prevent it from deploying its large army. When defending, it tries to block the archers, crowding them together, even though its cavalry and seraphs can wipe out entire stacks of enemies in one blow.