E1 styled, an abbreviation of Episode 1 styled, is a common expression used to describe the gameplay or visuals of a map relatable to the designs and principles employed in the Knee-Deep in the Dead episode of Doom. It is a specific sub-trope of the tech base style.
E1-styled gameplay generally refers to a focused monster variety, usually limited to Zombiemen, Shotgun Guys, Imps, Demons, and Spectres in staggered numbers throughout the level(s). It also refers to an arsenal that is primarily focused on the Shotgun and Chaingun. Rocket launchers, Chainsaws, and pistols are included too, but generally Berserk packs, Plasma Rifles, and BFG9000's are scarce, as in canon, they don't appear until Episode 2: The Shores of Hell and Episode 3: Inferno. E1-styled gameplay may also be inclusive of the level design principles, such as basic monster closet traps, lightly damaging floors, non-linear layouts, and consistent run-and-gun gameplay, involving large monster quantities, a surplus of ammunition, and generous health pickups.
The most dominant features of a visually E1-styled level are the STAR variety of textures with the rust streaks and bullet holes. Combining such with sections of computer panels, mechanical tech walls, metallic hexagon tiles, nukage spills, brown concrete walls with leaks, and the famous background mountains and sky often completes the aesthetic. E1 styled visuals may also refer to the simplicity in the maps design, with limited details, angled and asymmetric walls and room shapes, height variation, large secret areas, and other features akin to John Romero's level design tropes.