When you start the level, go through the doorway and kill the grunt (optional) and crouch-jump onto the light. Turn around, go onto the ledge and jump to the hallway thing. There will be an invisible force-field. Grenade jump to the lowest ledge closest to the force-field. Jump on the white roof and go to the hallway that narrows and jump on the object in the middle of it. Jump on the roof and turn right. Jump on the bridge then jump on the railing and crouch jump onto the building. Go straight and you will see a red roof to your right, jump on it and go up the ramp. Jump on the vents then to the roof to your right. Go straight then to your left and you should see a hole in a red roof where a bomb might of hit, jump into it.

~SPOILER~

You will see a energy sword in the ground and the name "REX" spelled out with rocks. Rex is one of the game designers.