You trigger a door by stepping on a floor switch. Inside the door, high up, is a target. Shoot it. It opens a door to a lift - ride it up. Jump down on the ledge and collect the secret.

Deep down, a squasher attempts to make you part of the wall decoration. Get behind it. Here is the slipgate to the next secret.

After securing acess to the gold key, the red mark glows. Shoot it. The stairs fold down. Run down to get the goodies. Now, STAY!

...And when the stairs rise again, a second secret is available! Enter the slipgate and get your reward.