Load the last checkpoint and walk until you see these double doors open, kill the enemies and go inside.

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On the right side of the middle door behind the boxes is a hidden switch, Press it!

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Walk through the now open middle door for the first secret level.

Load the last checkpoint and head to the last large room that looks like this. Head up the stairs to the room the crosshairs are on.

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You will see a bunch of barrels with light peaking beneath them. Destroy them and open the wall to reveal a passage.

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You will be in a "retro" room. Press the Use key on this middle wall portion the crosshair is on, and it will slide back to reveal the secret exit.

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Load the last checkpoint and head to the room right before the normal exit with the 4 robots that drop down. Look for this portion of the wall and press the Use key on the section the crosshair is on.

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It will open and reveal the switch to the secret level.

Load the last checkpoint and you will be in a room that looks like this. Head towards the wall the crosshair is on.

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Behind the armor is a fallen torch, walk behind the torch model and press the Use key on it.

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The large wall will open.

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...and it will reveal the secret exit.

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(You can get crushed by the flame wall, but it will only do 25% of your health in damage, so don't worry about it)