Doom/E3M3: Pandemonium

Collect the blue key. To do this, enter the area directly north of the start, and take the north-east exit. Follow the right-hand wall until you drop down to the lower level. The blue key is in the metal room that overlooks the starting area.

With the blue key in hand, head to the northern part of the map to reach the blue key door. There are three routes to the exit.
 * The first route is directly west of the entrance. It leads through some monstead.
 * The second route involves takine the north-west exit from the central area. This is tagged as a secret (see below for more information).
 * The third route follows the right-hand path. However, there is a Baron of Hell guarding this area.

Secrets
There are six secret areas:
 * 1) The most important is the north-west exit of the central room.  This leads to a BFG and a supercharge, both which are found in the nukage pit.  To claim the secret, reach the exit of the nukage area.  If you follow the stairs down to the bottom from the exit, you can also collect an invulnerability charge.
 * 2) In the eastern route, you may see a nukage pit.  Collect the chaingun in an alcover in this pit (as visible from the window above) for the secret.
 * 3) In the corridor in the eastern route, the north-east darkened section between the Lion faces leads to a small bridge.  Reaching the floor before the bridge is a secret.
 * 4) Continue along the path until you get to a red room.  There are two routes to reach the secret.  The first follows the path and required raising a sunken floor, and the other cuts across the nukage and collect the berserk path.  Either route will lead to a secret room containing a rocket launcher, a backpack, green armor and other items.
 * 5) In the north room before the exit, walk onto the raised floor behind the northern decoration.  This lowers a wall containing a few items, including a computer area map.
 * 6) In the north room before the exit, walk towards the south-east alcove.  This lowers the wall where you can collect some items.