Maga, are you sure you're in the right place? The workshop is down in the area you first come to after swimming through either statue pool.
There is another, similar door set between two torches, down one flight from the upstairs armory... but that's not the right one. (I think that's the wedged shut library door.)
My problem is I'm slightly short of loot. I have 3,159 out of 3,689, and I need 3,200, or 41 more. I think I've been everywhere... can anyone suggest a place I might look? Obviously, I've missed a cache (or scattered 5gp coins) worth a whopping 530!