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On "Warrior" difficulty, there shouldn't be any problems.
On "Veteran," if you can't gather enough resources to create artifacts that reduce spell damage, you won't be able to counter Armageddon and black dragons.
You'll have to give up the city of mages and capture the nearest dark city (there are no black dragons there, but you can still gather enough forces to repel the Warlock). Then, go through the portal to the north, capture the second city and the mines. After that, Zephyr holds the key position at the portal, preventing the dark forces from moving through the dungeon.
This is enough to win. The computer starts to clearly give up the game and doesn't even try to retake the third castle in the west.
On "Veteran," if you can't gather enough resources to create artifacts that reduce spell damage, you won't be able to counter Armageddon and black dragons.
You'll have to give up the city of mages and capture the nearest dark city (there are no black dragons there, but you can still gather enough forces to repel the Warlock). Then, go through the portal to the north, capture the second city and the mines. After that, Zephyr holds the key position at the portal, preventing the dark forces from moving through the dungeon.
This is enough to win. The computer starts to clearly give up the game and doesn't even try to retake the third castle in the west.