Besieged is a region control system added in the Treasures of Aht Urhgan expansion of Final Fantasy XI. The system is calculated on more regular basis than Conquest, which takes into consideration weeks at a time. Besieged deals with the Empire of Aht Urghan which lies on the new continent that this expansion has added.
Unlike Conquest, it is assumed that the beastmen are in an uprise against the nation of Aht Urhgan. The Mamool Ja Savages, Troll Mercenaries, and the Undead Swarm raise armies and raid the city of Aht Urhgan on a regular (yet unpredictable) basis. Players gain Imperial Standing Points by participating in these raids and also by killing creatures in Aht Urghan while having the sanction of Aht Urhgan.
Important to all of this is the Imperial Defense score, which measures the success of the players on each server at defending Aht Urhgan from its enemies. The higher the imperial standing, the greater the experience that is rewarded in new zones, the greater the imperial standing that is gained by players.
Sieges consist of protecting the Astral Candescence, a mysterious object which strengthens those whom possess it (i.e. If under Al Zahbi possession, there are bonuses to sanction; under beastman control, abilities like Sneak and Invisible of players are weakened). There are plenty of NPCs to help: five Serpent Generals, Imperial Troopers, Immortals, Volunteers and even a certain type of beastman mercenary (either Goblin or Qiqirn). The Serpent Generals (the most powerful of the NPCs) each hold a seal to the Hall of Binding. If all five generals are defeated, the hall opens and it's up to the players and the remaining NPCs to keep the enemy from stealing the Astral Candescence. If the Candescence is taken, the players must enter the enemy's lair and reclaim it through a boss battle of sorts.
Serpent Generals and Volunteers are kept as prisoners if they fall in combat. Players will be unable to perform certain tasks if the respective NPC is taken prisoner; players must enter the lairs of the beastmen to free them.
Conquest deals only with the balance of power between the beastmen hordes and Aht Urghan (a role that the player characters fill by acting as mercenaries).