Fortified Zone is essentially those top-down stages from classic Contra titles but with 2-player co-op over a link cable. It is impressive because each player can venture in their own direction with all their action happening off-screen. Honestly, I am not really sure how this is possible on the original Gameboy but it offers some excellent cooperative gameplay.
A Japan exclusive sequel was released a year later, Ikari no Yousai 2, and also offered 2-player co-op over a link cable.