Does the AI ever attack forts?

[Tamur]

They also provide a rally point for retreating armies. Putting up a fort before engaging in a battle or a siege can prevent your army from leaving the area altogether if you lose or chose to withdraw.

Probably the most useful thing about them, besides the basic walls from which you can sally. Having routed units regroup at the "flagpole" in a fort defence has saved my army more than once.

Forts are so cheap that I build a lot of them when in enemy territory. So did the Romans.

Hehe, yep, every night! The work involved doing that over and over is staggering...