The original FPS in a 2D side scroller, with over the top everything, from weapons to bosses. I really love these games which in the collection from the original Metal Slug to Metal Slug 6. The only real difference is in Metal Slug 6 you can't save your progress. The game has to be played in its entirety all at once. Hell, you can make any of these games as difficult or as easy as you want. No continues to unlimited. They are pure fun!!!!
Oh, sure I could get into how these game depicts combat and saving prisoners. But why bother all the prisoners look the same, and these games are both fun and challenging all at the same time.