Duke Nukem 3D/Occupied Territory
You start the level facing a locked door. Turn and face the wall behind you. You can collect the shotgun and extra shotgun ammo, and then use the switch to open the door. Proceed through the opened door.
In the next room, a door on your left will begin to close as soon as you enter the room. If you move fast enough (Hint: moving diagonally is twice as fast), then you can actually pass through the closing door before it locks you out. However, you may find it useful first to collect the portable medkit and chaingun cannon in this room before exiting through the door. If you are locked out of the door, then simply enter the control room on the right and activate the switch to reopen the door. A portable medkit can be found in the control room, and a chaingun cannon can be found in a compartment in the wall in the center of the room, outside the control room.
Upon reopening the door, you can collect an atomic health - and the level's first secret - by standing atop the opened door, crouching down, and crawling into a hidden crawlspace on the right side of the door.
This next corridor is packed with enforcers. Upon exiting this corridor, you will enter a much larger corridor with assault commanders guarding the door at the opposite end. When you cross the midpoint of this larger corridor, sentry drones will be released from a hatch above the door behind you.
Because there are so many sentry drones, it may be useful to know that you can easily kill the sentry drones - and conserve your ammo - by exiting through either door, closing the door behind you, and standing up against the closed door. If sentry drones come within close proximity, they will explode, even though you are on the opposite side of the closed door and cannot be hurt by their explosions.
Although it appears as though you can fall into outer space, you can actually walk up the glass ramps on either side of this large corridor. At the top of one of these ramps - specifically, on the right side in the second half of the room - you can find a vent in the wall in the middle of the corridor. The RPG inside this vent may be helpful for fighting battlelord sentries later in the level. If you still have a jetpack from the previous level, then you can also collect RPG ammo and an atomic health from within the hatch that released the sentry drones. Also, shotgun ammo and large medkits are hidden along the sides of the ramp that was guarded by the assault commanders.
Proceed through the subsequent red room to reach a massive circular lobby at the center of the level.
IF YOU HAVE A JETPACK FROM THE PREVIOUS LEVEL:
Upon exiting the red room, collect the steroids on the left side of the exit ramp. Activate the steroids, and run diagonally up against the door directly in front of you (you normally need a blue key to open the door). While running diagonally into the door and while your steroids are still in effect, quickly mash the jump and crouch buttons on your keyboard/controller in order to glitch through the door. This glitch works because Duke actually sinks beneath the plane of the floor whenever he lands from a jump (the developers deliberately programmed this into the game in order to make it seem like Duke's knees were recoiling whenever he landed). Although Duke only remains below the floor for approximately one frame, crouching during this frame allows Duke to pass through thin doors at various points throughout the game. Slightly thicker doors can also be crossed if Duke is moving fast enough, which is why you need the steroids to glitch through this door. Note that there is a small chance that you might be squished by the door when executing this glitch, especially if you aren't moving at a perpendicular angle with respect to the door.
You can use your jetpack in the next room to activate a hidden switch on the wall near the top of this room. This switch will open a door on the opposite wall, revealing a green exit button that will take you to a secret level. Because this trick skips so much of the level, there is no way to reach the green exit button without a jetpack. In fact, you will be trapped inside this room and must restart the level if you don't have a jetpack.
IF YOU DON'T HAVE A JETPACK:
Upon exiting the red room, take a left turn to enter a green area with protozoid eggs and shrinker ammo. You should come to two fast-moving conveyor belts. An invincible turret at the opposite end of these conveyor belts will be firing in regular intervals. While dodging bullets from the turret, simply run down the conveyor belt to the opposite side. Walk around the wall with the mounted turret to collect the devastator and the red key on the opposite side. You can also find pipe bombs in the corner. On the return trip, enforcers will try to ambush you.
Upon exiting the green room, proceed to the left. Use the red key to open a ramp that will allow you to reach the door above you. In the room at the top of this ramp, you will find one or more battlelord sentries, depending on the difficulty setting. Battlelord sentries are easiest to kill by firing the shrinker at their crotch areas. There are no shrinkers on this level, so this can only be done if you still have a shrinker from previous levels. Get the blue key in this room and return to the central lobby.
Use the blue key to open the door(s) in the central column. Enter and activate the switch in this room. Return to the lobby outside. On the side of the central column that faces the green room with protozoid eggs and conveyor belts, you will notice that a door was opened.
The opened side-door leads to the upper deck of the central control room. Activating the switch at the top of this control room will simultaneously extend a bridge and also reveal two more battlelord sentries on the opposite side of the bridge. Again, if you have a shrinker from the previous level, firing the shrinker at the battlelord sentries' crotch areas is the easiest way to kill them. Even without a shrinker, these battlelord sentries are relatively easy to kill since they are trapped, and you can easily fire at them while alternating behind cover. After killing the battlelord sentries, cross the bridge, and the level's exit button can be accessed via a switch on the right, behind one of the dead battlelord sentries.
There is also a second exit button that will take you to a secret level. To access it, walk to the center of the bridge. While facing the battlelord sentries, turn 135 degrees to the left (roughly 7 or 8 o'clock). Look up. You should see a button very high up on the wall. If you shoot the button, then a door will open behind you. Jump from the bridge to the opened door to reach the secret exit.
- When you open the first blast door, crouch down and enter the crawlspace to the right for an atomic health.
- In the central area, there is a grate on the central separator. Kick it open for armor and an RPG.
- In the central area, enter the point where the sentry drones arrive from to collect RPG ammo and an atomic health. This secret seems to require the jetpack, which is not located on this level.
- In the room where you get the blue keycard, the broken screens can either be opened or passed through. You can collect atomic health and two laser tripbombs.
While not counting as an in-level secret, you can reach the secret exit by shooting a button near the ceiling in the final part of the map.