From the entrance, go left. There is a teleporter in the middle of the blood. It leads to the middle of the secret.

Get the blue key. Turn left inside the blue door and go through the green rooms. In the room on the pic, open the small door to the right. It has a lava floor and a switch. Go to the back, press the switch and get the goodie.

In the room directly across from #2, go to the wedge-shaped end of the room and tap it. Secret. Hm. Perhaps those exploding barrels can be of some use...?

Get the red key. Right inside the red door, 3 'notches' with bad guys on the right. At the bottom of the third, tap the wall: secret door. You will need the goodie inside.