A lot of people seem to be pointing at Arcus and frankly I doubt it, mainly because Arcus is a nation(?) that is in the sky and I imagine the transportation method from Arcus to empire would have entailed teleportation or flight. The method of a carriage seems a bit too primitive for mages and considering the mages were after the princess to confirm if she is their leader it would be premature to just kidnap her. Also putting a anti-magic collar on her seems counterintuitive to the objective as well since it would be pretty damn rude to cut your boss off from her magic.
Though then again if it is the mages, we the audience won’t need to bully the guard for being trash so...eh.