You fell into the basement after a walkway broke, have shot your way out and has climbed out through a collapsed floor to the crypt above. Between a couple of coffins, a stone sticks out in the back wall. Press the stone, and run out on the remainder of the walkway.
On the left, at the bottom of the 'T' of the walkway, a secret door is open for only a few seconds.

Please pay attention to the ornaments on the wall over the 'goodies' - they are the key to the next secret!