0'Brothers was created as an exercise in creating a very Super Mario Bros.-like mappack. As such, it does not feature any elements from Portal, aside from the Portal Gun itself, now a mandatory tool for traversing the levels.
The mappack features stunning, 8-bit styled graphics, including great Tiles and detailed Backgrounds. It even has custom music, with an optional mod to replace the default songs with yet more custom tracks.
The mappack spans a full eight worlds of four levels each. The gameplay is noticeably more difficult than Super Mario Bros., and features many puzzles with portals, even to the point of infamy regarding Koopa shells.
0'Brothers has been widely regarded as one of the most well-made mappacks for Mari0 in terms of visual design and creativity. However, much criticism has been leveraged against its overuse of shell puzzles. Many forum members posted about only getting a few levels in before the mappack just became tedious to play.
Despite this, it is still one of the most downloaded and well-known mappacks on the forum to date.