The Doom engine is the game engine that powers the id Software games Doom and Doom II, as well as Raven Software s titles Heretic and Hexen, then finally wrapping its final commercial license with Rogue Entertainment s Strife. It is a sector-based engine.
A map that is set in a city that was invaded by Korax s monsters. You are free to do anything you want in this city, whether that is exploring each building, killing monsters, etc. Deathkings of the Dark Citadel is not required.