Fortress is the eleventh mission of Metal Slug: 1st Mission.
The Hero infiltrates a Rebel fortress. The Hero starts on the outside and later traverses the inside by riding elevators and dodging fire. A hallway that splits the fortress is navigated by stepping on a switch and jumping on timed blocks over a pit. The second half of the fortress has a pool of water with piranhas. Near the fortress' exit, the Hero can take an alternate route by jumping over a timed-spiked platform and riding a moving platform that opens occasionally. If not, then the final stretch is an ambush with Rebel soldiers.
- Complete Mission 10 (Night Flight).
- Complete Mission 14's Prison Camp (Weapons Port).
- Mission Complete: Proceed to Mission 12 (Weapons Dump).
- Alternate Route: Proceed to Mission 15 (Breakthrough: With Prejudice).