The game started as a simple pet project to learn the system: Fries programmed a Master Chief sprite, had him move and shoot. But instead of calling it a day, he pressed on to make it a full-fledged game using the same limitations of most Atari 2600 games: create it using just 4 kilobytes of data. The game was to have a level that featured a driveable Warthog, but not only did it push the size of the game over the 4K limit, according to Fries it also wasn't that much fun to play. So he scrapped it.
Halo 2600 has a very "Adventure" feel to it: players control Master Chief in a non-scrolling exploration design. There are 64 "rooms" to explore with a variety of enemies to shoot, including a final boss – but first you have to find your gun. Energy barriers can impede your progress, but once you track down a key or take down enemies the wall will disappear and you can move on.