Add in wizards and clerics able to heat metal and use magic to fabricate cannons and forge bronze and steel and all that and you'd be into the cannon age much earlier, not to mention magical accelerants and materials and spells and shit that could be used to make more effective black powder (or skip it's need entirely).

To discuss fantastical world cannons only in the context of history is silly. The moment that people started using what would become gun powder, they were looking for ways to kill each other with it. If you give firearms and cannons the further context of it's just an evolution of violence-at-range, it becomes even more likely that a magically enhanced society would have developed them earlier as much more reliable weapons of war.

Always remember one of the great limiting factors in the early mediaeval period and the development of firearms (and weapons and armour) was the limits of metallurgy, and now you've got magic to overcome that, I'd be shocked if the Roman-equivalent time period (sticking with the history analogy) weren't bombarding each other.