A fortress is a small area similar to a clan hall, can be either small or big. In order to capture a big fortress you need a warsmith to turn off the power station. Fortress siege lasts for one hour and all siege items can be used (elixirs,scrolls,etc).
There is a list to help you distinguish a small fortress from a big one :
White Sands Fortress
Cloud Mountain Fortress
Once the fortress siege starts, Mercenary Captain will appear in front of fortress’ gates. This NPC must be protected from the attacking clan in order to open the doors from main building and get a flag. When you enter the fortress you must kill first three bosses. It would be good to kill them in a particular order and within 10 minutes to avoid respawn.
First of all kill the Guard Captain, don’t worry about guards around him, just ignore them since they will disappear when Guard Captain die. Try not to aggro him out of his room, leaving his room will fully restore the boss.
There are two more bosses to be killed. It is a matter of choice which one will be first. Mage boss is generally easier, so i would first choose this one. Kill healers first and then kill the boss. Just be careful and try to kill it fast to eliminate the risk of using AOE nuke.
After killing the mage boss, head to archer boss straight ahead. This boss is a bit harder and stronger than mage boss, that’s why a tank would be useful.
When the three bosses are dead, if u participate in a siege of a small fortress the doors of main building are open and u can capture the flags and run to the roof. If the fortress doesn’t have an owner, there will be a boss in the roof. You can just kite the boss away and people with flags start casting.
In case the fortress is big, after the three raid bosses are dead, you need a warsmith to turn off the power station. You just need few Control Room Cards, that can be obtained by Dwarven Gunnery Sergeants around Control Room. Meanwhile you should start killing the npc called General inside Officer's Barracks. When power station is off and General dead, doors from main building will open to get a flag and run to the roof.
Once you capture the fortress, two or even three passive skills will be added for all clan members.
There is a list with skills from each fortress :
Aaru Fortress - Residence Soul, Residence Clarity.
Antharas Fortress - Residence Empower, Residence Shield Defense.
Archaic Fortress - Residence Body, Residence Vigilance.
Bayou Fortress - Residence Might, Residence Empower.
Borderland Fortress - Residence Death Fortune, Residence Spirit.
Cloud Mountain fortress - Residence Death Fortune
Hunter Fortress - Residence Evasion, Residence Might.
Demon Fortress - Residence Spirit, Residence Moral.
DragonSpine Fortress - Residence Magic Barrier, Residence Resist Typhoon.
Hive Fortress - Residence Agility, Residence Resist Lava.
Ivory Fortress - Residence Moral, Residence Freedom.
Monastary Fortress - Residence Body, Residence Health.
Narsell Fortress - Residence Body, Residence Vigilance.
Shanty Fortress - Residence Body, Residence Shield Block
Southern Fortress - Residence Resist Typhoon, Residence Shield Block.
Tanor Fortress - Residence Movement, Residence Resist Lava.
Valley Fortress - Residence Clarity, Residence Health.
Western Fortress - Residence Death Fortune, Residence Shield, Residence Shield Block
White Sands Fortress - Residence Soul, Residence Residence Magic Barrier.