This is a story-related achievement, but one cannon might be easy to miss. It's in the underground caverns part of the level, as you're flanking the cannons above ground, toward the end of the underground bits. The natural path of the room will be to the right and up a few ledges, but there will be a platform hanging to the left which you can use to jump up to a ledge on the left where a cannon is shooting up through a hole in the ground. Listen for the sound of cannon fire as you enter each underground area and you'll find it.
The rest of them are up on top, on ground level. I'm not sure if there was 3 or 4 of them, but there's not that many.