In the room where you get the blue key, cross the eastern bridge and follow the way until the next door. Push a part of the opposite (northern) wall to find a Invulnerability sphere (Sector 120).
To get access to this secret, you need to acitvate three switches which are hidden behind pentagram stone walls. (a) At the start of the map, turn around to find the first wall. (b) After the blue key door you will find the second one. (c) Go through the door south of Secret 1. After the western door on the southern wall you find the third one. After using the teleporter behind the red key door, look to the north to find the (now unlocked) path leading to the switch that lowers the pentagram stoned elevator east of you. Use the elevator and follow the way to the area (Sector 166) around the secret exit, which counts as secret. You can now use the secret exit to map 32 or reenter the elevator and complete the map using the regular exit to map 16.