DOOM II: Hell on Earth is a computer game developed by id software and released on October 10, 1994. It is the sequel to DOOM, id's seminal first person shooter released in 1993. It was originally written for the PC using DOS but has since been ported to Linux, MS Windows and GameBoy Advance. Due to its heavy graphic violence, it has an ESRB rating of "M". DOOM II marked id's departure from the shareware marketing strategy, releasing this game through Activision.
DOOM II consisted of thirty regular levels and two "secret" levels, which nearly duplicated episode 1, level 1 and episode 1, level 9 of Wolfenstein 3D. The engine and gameplay were the same as DOOM, however there were some different weapons and monsters. The two secret levels were missing from the version marketed in Germany because they depicted Nazi symbols, illegal under German law (see the article on Wolfenstein 3D for more information).
Unlike most id games, DOOM II didn't break any new technological ground. Since it consisted of just new levels, new enemies and a few new weapons, but used the same game engine that powered DOOM, the game was seen by fans as more of an expansion pack to DOOM rather than a sequel.