User blog comment:Raylan13/Elder Scrolls Online Community Q&A/@comment-4662333-20140319053414/@comment-13446185-20140320201333

They definitely aren't common but read this;

"Why was the Sentinel army so useless during the War of Betony?

The cannons were too heavy, so all three garbage scows sunk."

-Jokes

Cannons are also seen on ships in. It appears to me that cannons are probably more of a naval combat weapon in TES.