Open the console by pressing the key just left of the ''1'' button on the keyboard; then type ''map x'', where x is any of the following:

start - the introduction level, in which you select difficulty and which episode
e1m1 - The Slipgate Complex
e1m2 - Castle of the Damned
e1m3 - The Necropolis
e1m4 - The Grisly Grotto
e1m5 - Gloom Keep
e1m6 - The Door to Chthon
e1m7 - The House of Chthon
e1m8 - Ziggurat Vertigo (a hidden level)
e2m1 - The Installation
e2m2 - The Ogre Citadel
e2m3 - The Crypt of Decay
e2m4 - The Ebon Fortress
e2m5 - The Wizard's Manse
e2m6 - The Dismal Oubliette
e2m7 - The Underearth (a hidden level)
e3m1 - Termination Central
e3m2 - The Vaults of Zin
e3m3 - The Tomb of Terror
e3m4 - Satan's Dark Delight
e3m5 - The Wind Tunnels
e3m6 - Chambers of Torment
e3m7 - The Haunted Halls (a hidden level)
e4m1 - The Sewage System
e4m2 - The Tower of Despair
e4m3 - The Elder God Shrine
e4m4 - The Palace of Hate
e4m5 - Hell's Atrium
e4m6 - The Pain Maze
e4m7 - Azure Agony
e4m8 - The Nameless City (a hidden level)
end - Shub-Niggurath's Pit

For example, type ''map e1m2'' to access the ''Castle of the Damned''.