Sadness
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Juristis, I will do just that.
There are several ways to fix this. You can completely prevent the use of the opponent's army, i.e., by limiting the growth of the main city. You can also limit attacks on the city to x-x-1. In other words, you can attack anywhere, but not the city. The same applies to blocking the main blue hero's path to his main city for x-x-1 turns.