I noticed this when I was attacking a city. One turn after my units go into position, a cannon magically appeared in the city. Since the AI had a lot of money this happened a few times. On a side note, when I moved my troops back (not wanting to get destroyed by the cannon) it decided to move the cannon out of the city right next to my troops allowing me to easily destroy it. I think it was trying to move the cannon back to its main empire... by going though my recently aquired territory. As a side note, after I defeated the AIs army, it was willing to give me everything it had for peace, including the 15 or so cities it had.