Ahriman
Tyrant
The problem isn't that the AI wont' use the bombardment when it is defending; that is the easy part. The problem is that a human controlled fort just *destroys* an invading AI.
The invasion AI picks a city target, and sends a stack towards it. It ignores bombardment along the way, so it will walk right past a fort (taking a lot of damage), up to a city, and start using its catapults to knock the walls down. By this time, the stack has been severely damaged by bombardment (units are at half strength or less) and the human player can easily destroy the entire invading army without losses.
The invasion AI picks a city target, and sends a stack towards it. It ignores bombardment along the way, so it will walk right past a fort (taking a lot of damage), up to a city, and start using its catapults to knock the walls down. By this time, the stack has been severely damaged by bombardment (units are at half strength or less) and the human player can easily destroy the entire invading army without losses.