It just doesn't look like any sort of cannon/tower. Working without reference or concept can leave your models looking really bland. I mean something like this looks more like a toxic cannon, if you just change the light to look more like a toxic cannon. Try to do some research and come up with a design you like before you…
Your cannon is missing the trunnions and the mounting brackets. If you look closely at the reference, you'll notice that the verdigris is much heavier on the top of the cannon, and falls off sharply below the mid-point as the metal curves in. You're also missing the inscriptions & the equestrian.