As ballista vs cannon towers.
Both can be effective but in different circumstances.
eg If you have cannon towers (well for those of us who haven't fixed bug, I'd better say towers that fire cannon balls) they can be devastating if you sally forth - they can hit targets further away than ballistas and do more damage. (I say sally forth - I usually go and make a cup of tea and wait for the cannons to demolish 40-50% of the enemy army so it runs away.)
Whereas ballistas seem to do a better job at siege equipment that is approaching and at close range.
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