Other than the obvious offensive uses for artillery, they can be extremely useful in defending a chokepoint. If you fortify 3 or 4 artillery(or cannons) along your "Maginot Line", as soon as war begins, start bombarding improvements to knock out all roads in their territory leading up to your fortresses. After this is done, you will get at least one, maybe two shots with your artillery at each stack of units that approaches, and then another if they actually make it close enough to attack. Plus, if they are launching a major offensive, you will have at least one extra turn after seeing it coming to reinforce your positions.
