On level -28, open the yellow door on the left side and speak to the dwarf beside the Water Elemental. There's a secret passage to his right.
Merchants in this Zone sell 5 Yellow Keys for 600, a Bomb for 400, and if you are not playing Casual Mode, one will purchase your Yellow Keys. Don't sell your Yellow Keys for gold unless you are REALLY desperate.
The Altar room on -25 contains for sale: Bomb, Magic Mattock, Scroll of Teleport, Scroll of Height, Golden Key, Water of Life, Wand of Flight, Crystal Ball, Vampire's Fang, Cross, Lucky Coin, and Ring of Antimagic.
Occasionally an Elder in this Zone will offer you a choice. He gives three options:
3: "I have come to destroy the Lamp." You receive a Water of Life.