The AI is decent at static defense - keeping a fair number of strong defensive units in its cities that require large attacks and seiges. But it can't mount a dynamic defense, attacking incoming armies before they chose a target.And the problem is that a static defense must be strong enough at all points to withstand a human player's concentrated attack. But if the AI is only slightly ahead of the human player, the human can overwhelm the AI at any particular point, and then pick the cities off one by one. In order to survive, the AI would need five or six times the units of the human player.
Solutions are two fold:
1. The AI has to be better at forward defense and destroying incoming armies before they reach the city walls.
2. The AI needs more units to defend with. This means bigger AI production bonuses. Note that even an AI that the game thinks is twice or three times as strong as the human based on numbers of units is still effectively weaker. AI production advantages can't drop out until the AI is substantially stronger...
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