Border Down is a horizontally scrolling shoot 'em up video game developed and published by G.rev. It was released in Japanese arcades in April 2003 on Sega NAOMI hardware, and was ported to the Dreamcast later that year. The story takes place in the future where humans are defending their Mars colony from an invading alien attack. The game employs a "border system" where each stage has three variations of different difficulty. The player starts on the easiest variation, and is lowered to a more difficult variation with each subsequent loss of a life.