You can tour the grey central part of this floor which way you want. Anyway be prepared for being attacked from both sides.

The Silver Key is found behind a drain on the east side next to one of the four secret areas. Find the Gold Key in the room in the south-western corner of the floor.

Push the block with the colored glass image to reveal the gold key door and enter the central room with the elevator.

Look for extra powerups and treasure in the four secret areas.