From the start, enter the first door on the left. It seems to be some kind of undertaker's workshop. Real cosy. Get on the very top of the shelves, a door opens in the other end of the room. Rocket jumping may help.

The stepladder in the picture is only present at 'Easy' setting. Save the whales... ehh....n00bs?
Thanks to Almos for telling me!

In the same room, one of the coffins have a skull plate inside. Step inside, the plate is a slipgate to some goodies.

Don't bother with the steps, stay on the ground floor. Take the air lift up to the lever (it allows you access to the Pentagram), but don't use it yet. Walk around on the stone beams, a door to some goodies will open.

Return to the lever. It may be just the right moment to shoot the bugger guarding the Pentagram.

This is a gold-plated bastard! Get the Pentagram, then race along the corridor, into this room... and jump overboard! I hope that Pentagram works. Make your way to the wall right under the entrance door. All you get for your trouble, is a corny message and something for patching your butt.

In the paranoid stonemason's extremely well-protected workshop, you will find a heap of raw headstones. Climb the pile, and get a goodie.

Get out of the opening to the right of the gold key. Carry on along the slanted edge over the blue door. This is a bad time for contemplating your fear of heights. When you reach the bit that seems turned 90 degrees, it will open. Good stuff awaits you.