Sure, but those usually have a gun shield protecting the gun crew & it is done due to the gun barrel being long & the gun being heavy, so it is better to mount it on a single point, instead to jave it traverse on a curve (which is not a problem for an archer).

Did 18th century cannons have gun shields?

My main reference are the pre WWII Czechslovak border defenses - these do have gun shields & apertures that get wider to the outside:

https://commons.wikimedia.org/wiki/Category:85mm_kan%C3%B3n_... https://commons.wikimedia.org/wiki/Category:4cm_kan%C3%B3n_v...

Not that the apertures are stepped instead of smooth - I guess the avoid the funneling effect mentioned above, possibly changing a near miss to deflection and hit on the gun shield. Should not happen if the steps get in the way.

Apparently not [0]. IANA cannon expert but I think in the 18th C their biggest threat would have been from cavalry rather than sharpshooting infantry or precision fire from other cannon.

[0] https://www.google.co.uk/search?q=18+century++cannon&tbm=isc...