I noticed in my last game that if I cast pillar of flame on a group of enemy units and severely damage it - then the AI is quite happy to throw itself at my heavily fortified city and die in a complete walkover. It seems as if the computer does not reevaluate the decision to attack the city although it has now become a clear suicide mission.

Also I noticed that after a pillar of flame some units stay alive for (at least) one turn. If the computer attacks immediately, the affected units will actually show up in tactical combat and have their first move (e.g. to fire arrows) before dying on their own as their first tactical turn ends.