Project 4 Fortress is the last stage of the SNES port of U.N. Squadron.
Details[]
It appears to be a hole in the jungle with a hidden fortress inside it. A lot of enemies spawn inside the fortress. When the player enters the fortress, It gets narrow at first. Then it becomes wide. There are missile launchers at the ceiling for the player to destroy.
Then, a rock in the center splits the path in two, but then it gets normal. The same thing happens after a few inches. Then, it transforms into metal plating.
Then it turns into a vertical maze. On sides of the maze walls, The player can see gun turrets. The player can destroy them.
Then the maze ends to a wall with rails. Gun turrets on a rail device will shoot to attack the player.
Then the whole fortress gets narrow again. The player enters a black void with a metal floor, Where the player faces Heavy Bomber.
After the player defeats Heavy Bomber, Project 4 Airship will appear, making the background look like a black void.