Maelstrom

Maelstrom is a vertical scrolling shoot 'em up game written in 68000 assembler. You control a spaceship and have to battle you way through asteroid fields and waves of enemy spaceships. The length of each level is determined by how fast the player kills the enemies; for every killed enemy the energy meter to the right on the screen goes up a bit and when it reaches 100% the player has completed the level. As the game progresses levels takes longer and longer to complete. When a level has been completed the earned points for the level is added to the players score.

The game comes with no background story, but level names such as "Border patrols" suggests that the designer had some kind of story in mind.