I was under siege by Milan in my Citadel with a small garrison. I had a few bombards, but the Milanese had about 10 pieces of siege equipment. I was counting on my cannon towers to help rip up their siege equipment.

Nothing.

My towers did absolutely nothing. So, I tried to lure them closer, and a unit of crossbows fell for the bait. What did the towers shoot at them? Ballista bolts.

I definately have cannon towers for my citadel. What's the problem?

Also, on a side note, huge walls for cities seem to fire cannons from the towers regardless of the tower upgrade. Strange...