Mars

Mars is the fourth planet from the Sun in the Solar System. It is where The Ultimate Doom episode 4 Thy Flesh Consumed takes place on. Mars' moons, Phobos and Deimos, are also used in the UAC. Episode 3, Inferno, takes place in hell, another location/part of Mars. Nearly all episodes are in close locations from Mars (from The Ultimate Doom):
 * Knee-Deep in the Dead-- Phobos
 * The Shores of Hell-- Deimos
 * Inferno-- Hell
 * Thy Flesh Consumed-- Mars

From the Doom movie, I remember there was a portal taking someone to Mars.

Also it takes WAY longer than expected to go to the outer planets from Mars, and even longer for planets closer to the Sun, due to an asteroid belt in-between Mars and Jupiter.