MAP01: Entryway is the first map of Doom II. It was designed by Sandy Petersen and uses the music track "Running From Evil". The par time for this map is 30 seconds (retroactively, because it's the first level).
You start the level at a ledge. In front of you are four Zombiemen looking back from you (be careful on higher difficulties as even a punch will alert more Zombiemen in the hallway). Kill them and proceed to the hallway. Continue down the path until you see room with Health bonuses, then turn left. You'll come to dark room. On the right side of the room is the exit. Flip the switch inside to complete the level.
Trivia and other points of interest
At the very start of the map, before climbing down to the floor, head southwest to access a partially open-air area and get a Chainsaw.
At the northwest of the winding corridor, there is a room containing eight health bonuses.
In the blue room at the north of the level, flip the switch on the right. A compartment will open in the northeast of the room, containing an Armor (which can be useful in next level) and two Stimpacks (sector 41). Toggling the left switch will cause both switches to sink into the ground, allowing the Zombiemen's clips to be acquired. If you do this, you can still open this secret by pressing the Use key where its switch was formerly located.
In the large room at the end, toggle the switch to the west to activate an elevator. At the top, open the back wall of the elevator to reveal another switch (sector 8). Be warned as on higher difficulties there are Imps here. Flip it to open a door in the southeast leading outside.
From secret #2, run outside. A compartment containing a rocket launcher will open behind the Health bonuses out in the hallway (sector 57). You do not have to jump onto the platform; all that is required is that you jump over the platform's west side (linedef 242).
In the big room, one side of the wall to the east is perfectly straight. On the north end of this wall is a cage containing an Imp. Open the other end of this wall to find a Stimpack (sector 28). On HMP or higher, two Imps will exit this secret if you fire your weapon inside the room or at the end of the corridor that leads into it.
Outside of the map along the south and east walls is a texture of white pebbles and rocks that is not observable without noclipping outside of the map (see WhiteTexture_2011.08.12_01.25.59.lmp below).