If the Alliance Fortress’s durability becomes 0, it is destroyed. 

Also, when the Alliance Fortress is destroyed, some of functions are lost.

■ Functions that will be lost with Destruction of Alliance Fortress

- Alliance Cannon Tower nonfunctional

- Unable to store in Alliance Depot

- Researched Alliance Buff nonfunctional

- Alliance Skill nonfunctional

- Alliance Walls nonfunctional

* Resources stored in the Alliance Depot is sent back to Alliance members.

When the durability is below 100%, you can repair the Fortress.

Make sure to repair it before it reaches 0.