Hell

Hell is the genre of The Ultimate Doom. There are many sections where the four episodes take place on, each being considered due to the name of each level (except for Episode 4, it does not mention the name of a building, instead it has direct quotes from the King James Bible).

Where each episode takes place on

 * Knee-Deep in the Dead takes place on Phobos, a moon of Mars, and due to it probably being the biggest UAC locale, computer textures are very common.
 * The Shores of Hell takes place on Deimos, which in the text screen at the end it said that Deimos floats above HELL, which is sometimes said as Mars itself.
 * Inferno actually takes place on Hell itself. Mentioned by the name of E3M1.
 * Thy Flesh Consumed indeed takes place on Mars. Obviously because the sky is similar to the sky of Mars. Now I confirmed the first level to take place on Earth is Doom II's MAP01.