I agree, this cannon is steam-punk garden decor. Besides the crude longitudinal "weld" line (a total no-go for a functioning artillery tube) the breech-area controls are obviously non-functioning -- being of one contiguous casting they can't spin.
All said, a very cool steam-punk garden decor well worth "restoring." I would start with a good hi-pressure washing (16psi or so) with a home pressure washer, if you have to borrow one. Next step, if necessary, would be a high pressure steam wash that a local garage might be able to provide. Last resort would be a sand-blast, but if this is made of relatively soft "pot metal" that would erode the surface too much.
If after a good scrub the surface is still not attractive, the thing can be painted with metallic paints, slightly different shades of brass and steel for the separate parts would appear most "convincing." If the last step is to paint anyway, I would fill in and smooth the entire weld line on the tube before painting with a hard spackling or automotive "bondo."
This sounds like a fun project. You know what, forget all that I'll be over in a couple of weeks to get it started. What's yer old man prefer in a beer?