You can find the Alliance Fort by following these steps: 

-Click on the (World) icon at the bottom left of the screen.
-Click on the (Map) icon also at the bottom left of the screen.
-You will see a set of icons indicating different locations (such as your base, alliance leader, alliance members, alliance castle, Battle Lab, and other places on the map). Click on the Alliance Fort location icon on the map, and you will be transported there.

Or you can press on the Alliance icon at the right bottom of the screen, then choose "Alliance territory." You will find Alliance Forts and their coordinates.