Quote Originally Posted by sapi
There's tons of bugs with the towers - what you saw was ballista towers firing cannons :P

Another notable bug is that huge cities get ballista and cannon towers for free with the default walls and lose them when they upgrade :P
That hasn't been my experience.

I fought a battle last night in a huge city that was in the process of but had not completed constructing ballista towers. The towers fired nothing but arrows. The city was attacked again a few turns later after the towers were complete, and they fired ballista bolts.