I can't figure out how to fire cannons in Castrum. Asking in pugs only results in people ignoring me or calling me noob.

So I get the ammo. Let the loader load it to the cannon. Click on the cannon. A red shooting icon appears in my skillbar. I make sure the huge yellow cycle is pointed at Livia. And then what? When I try to use that skill nothing happens. I'm playing on PS3, if that makes any difference.