User blog comment:FairyRaven/Fairy Tail Chapter 325: Unity, Review and Commentary/@comment-5444132-20130329113601/@comment-7418318-20130329141029

That the gate is actually not a cannon, but rather a door is official information.

Though the princess believes that is can also be turned into a cannon, mostly because future Rogue told her so, but I guess she would have consulted with Mages about the changes Rogue proposed to her.