The last Upgrade Altar in the game is on level -15. There are no more in Zone 5. So spend your gold here when you have enough to upgrade.
There's a secret level between level -16 and level -18. You can get there by using the Scroll of Height on level -18. The Genie here gives you four options:
1: Provides you information about the King.
2: Wakes the King from his sleep. You receive a Golden Key.
3: The Genie disappears, and you will have to fight him at the end of the game.
4: The Genie attacks immediately. You CANNOT defeat him unless you are able to defeat the final boss.
Merchants in this Zone sell a Golden Key for 1000, unlimited Yellow Keys for 200 apiece, a Scroll of Teleport for 600, and a Red Key for 2000.
The Altar room contains for sale: Bomb, Magic Mattock, Scroll of Teleport, Scroll of Height, Scroll of Earthquake, Scroll of Chaos, Wand of Flight, Crystal Ball, Vampire's Fang, Cross, Lucky Coin, and Cloak of Shadows.
The boss battle requires some precision. Kill the four middle minions (east, west, north, and south) to form an "X" pattern and make the boss killable. But if you pass too close to a minion, he may backstep, and the pattern will be impossible. To avoid this, steer around the corners of the room.