The server has two nations. The towns inside those nations act as strategic defense points for the nation. If a conflict arises, one nation can declare war on the other, enabling PvP and signaling the towns to either prepare for an attack, or prepare to aid the attack on the other nation.
The plugin we use to manage the wars allow for a certain amount of 'defense points' that each town has. Once a town's points reaches zero, the ownership of the town is moved to the attacking nation.