Twenty-one fortresses stand between the castles, and any of them can be taken in a one-hour battle at almost any time of the day. This page explains who may attack, how the siege goes from the barracks to the flagpole, what the owner gains and what the choice between independence and an oath to the castle changes.
The fortresses
A fortress is a clan residence like a clan hall, only it is won by the sword. Every territory has one or two. They differ in size and in place: a territory fortress, marked with a blue flag on the map, belongs to the land of one castle, and a border fortress, marked with a red flag, stands between several. The world information of the map (Alt+M) shows who owns each of them.
- Small territory fortresses: Camp Fortress (Shanty Fortress), Hive Fortress, Ivory Fortress, Narsell Fortress, White Sands Fortress and Archaic Fortress.
- Large territory fortresses: Gludio Southern Fortress (Southern Fortress), Valley Fortress, Bayou Fortress, Borderland Fortress and Swamp Fortress.
- Small border fortresses: Tanor Fortress, Dragonspine Fortress, Aaru Fortress, Demon Fortress and Monastic Fortress.
- Large border fortresses: Floran Fortress, Cloud Mountain Fortress, Antharas' Fortress, Western Fortress and Hunter's Fortress.
A clan may hold either a castle, or a fortress together with a clan hall. A clan that owns a fortress and takes another one loses the first: it goes back to the NPCs.
Registration
The leader of a clan of level 4 or higher registers with the Suspicious Merchant who walks near the fortress. The first clan pays 250,000 adena, and from that moment the clock runs: other clans have 50 minutes to join for free or to withdraw, then the merchant disappears, and after a ten-minute countdown the siege begins. The clan that owns the fortress is entered as the defender automatically. The same fortress can be attacked again four hours later, at any time of day or night.
How the siege goes
- The siege lasts one hour. The gates close, NPC guards take their posts, and the land around the fortress becomes a battle zone where the registered clans may put up a siege headquarters.
- Sides are marked as at a castle: a sword over the attackers, a shield over the defenders. Your own clan and alliance are friends; every other registered clan is an enemy and can be attacked without Ctrl.
- Inside stand the barracks, three or more depending on the size, each with its commander, and the command post in the middle. A barracks falls when its commander is killed.
- From the first fallen barracks you have ten minutes for all the rest. If you are late, the commanders return and everything starts again.
- A large fortress has one more task, the control room. Its power units have to be switched off, which is work for a Warsmith or a Maestro, and then a three-digit code is entered at the controllers; the access card is handed out by the dwarves next to the room.
- When every barracks is taken, the doors of the command post open and three flags appear inside:
Combat flag. Whoever picks one up carries it in his hands, cannot attack and walks slowly; if he dies, the flag returns to its place. - The siege ends at once when a flag is raised on the pole on the roof of the command post: the clan of the bearer becomes the owner.
Ballistas stand inside the walls. A clan member who destroys one with 
Ballista Bomb earns reputation for the clan, and everyone who fights in the battle zone receives Fame as the siege goes on.
Independent or sworn to the castle
After the victory an envoy of the castle appears in the command post for one hour, and the clan leader has to choose. A territory fortress is visited by the envoy of the castle of its land, a border fortress by the envoys of all the neighbouring castles. If the hour passes without an answer, the fortress stays outside both camps: neither its defence functions nor its dungeon can be used.
- Sworn to the castle. The clan pays the castle a tax every six hours out of the clan warehouse; when the money runs out, the fortress becomes independent. In return the defence can be upgraded and more guards hired for the next siege, and the logistics officer hands out supplies whose level grows every six hours up to the sixth. The dungeon of the fortress passes to the clan of the castle.
- Independent. No tax, no upgrades and no supplies, and the lord of the castle gets the right to register for the siege of this fortress. But the dungeon under the fortress is open to the clan.
What a fortress gives
- The services of a clan hall: restoration of HP and MP, a smaller loss of experience on death, teleports within the territory, buffs and the clan warehouse. Compare with Clan hall: auction, upkeep, decorations and contested halls.
Blood Oath. The logistics officer gives it to the clan every six hours of ownership, and the clan leader needs it to raise the clan to level 9: see Clans and alliances: creation, levels and structure.
Knight's Epaulette. They are earned from the guards and commanders during a siege and in the dungeon. The supply captain sells shirts, bracelets and talismans for epaulettes, and they can also be turned into clan reputation.- Clan reputation for the capture itself and for every six hours that the fortress stays yours, and residence skills for all members of the clan except the academy.
Ownership is not for ever: after some days the rebel army of NPCs takes the fortress back, and it has to be won again.
The dungeon under the fortress
The camp warden of an independent fortress lets the owner clan into an instanced dungeon. Every member needs the quest Awl Under Foot, taken from the same warden, and the party must have at least two players; the leader does the talking. A run lasts up to an hour and can be repeated in four hours. The bosses inside vanish if nobody attacks them within ten minutes of the entry. For the victory the party receives 
Dungeon Leader Mark, which the warden exchanges for Knight's Epaulette.
A castle has a dungeon of its own with the quest Blade Under Foot, and the castle itself is taken by other rules: see Castle siege: registration, rules and how a castle is taken.