A guard zone is used to describe an area under the protection of city guards, usually a town. While in a guard zone, if an innocent (blue) player is attacked or stolen from they can say "guards" and their attacker/thief will instantaneously punished in the form of a NPC guard instantly appearing and striking down the assailant in a single hit, accompanied by a bolt of lightning.
Guard zones can also include the areas around a moongates and even extend a ways outside of a town's distinct borders of buildings and structures. For example, the guard zone covering Britain reaches quite a ways eastward along the road and in the forested area.
In a few guard zones the rules of protection vary slightly. Traditionally, murderer characters setting foot inside a guard zone could instantly have guards called upon them, but during some events such as the Undead Invasion of Trinsic or, on some shards such as Siege Perilous or Mugen, a murderer is safe from the guards unless they initiate an unlawful act.