There are 4 fortresses that can be found. They are stronger and feature more defenses than outposts. However a fortress will become weaker when its ruler is killed or after you have taken down its alarms.

Location	How to unlock
Baghadur	X:196, Y:600. Ruled by Noore Najjar.
Rajgad Gulag	X:811, Y:795. Ruled by Pagan Min.
Ratu Gadhi	X:562, Y:604. Ruled by Yuma Lau.
Varshakot	X:511, Y:430. Ruled by Paul Harmon.