Open the door in front of you, as well as the northern door in the next room. Go down the staircase, and press the switch. Go up the stairway in the following room and grab the blue key. Head back up the stairway, and open the blue door to the east. Grab the red key at the end of the passage, and return to the start of the level. Open the red door to the west, and take the lift down. Open the door, head outside, and grab the yellow key at the east. Wait until the northern wall lowers, and then enter the passage again. Open the yellow door, and flip the switch.
From the place where you grab the blue key, jump to the square platform below. A door will open to the east, leading outside. This actually consists of two sectors, both marked as secret. (sector 48)