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, with 2D sprites representing objects, enemies and everything else that's not the map.
If I'm still playing a game nearly 23 years after it's release; I think it deserves a 10.