In Fort Apocalypse you fly a helicopter with which to try to rescue some people and destroy the enemy core reactor.
The camera angle is side-view and the map is cylindrical, meaning after flying to the right (or left) for some time, you get to your point of departure. Unlike a game like scramble, the map doesn't scroll itself, dragging the player along; the player determines his direction and thus, in which direction the map scrolls.
The map is divided in some levels, or better: layers: after shooting some tanks and other enemy stuff, you'll descend in a cave, doing more shooting and rescuing people, you'll find the way down to the next level. Some entrances need some thinking and puzzling before you'll get through.