Tower

The Tower is the optional level of the Catacombs.

You can access it by unlocking the door at the second level of the Catacombs with a Treasure Key. In this level, you can gather one of the four Rune Orbs (this one provides 2 SPD) in the game. Like in the Forest Temple, there is no spellbook in the Tower.

Unstable Platform
These wooden platforms begin to shake as soon as a player lands on them. After a second, the platform deactivates, sending any creature on it plummeting down.

The platform reactivates after a short while. The platform automatically deactivates underwater.

Void
Falling off the level results in instant death.

Countermeasures

Death Protection gear will bring you back to solid ground after you have died. This is not always foolproof.

Unlimited Jumps gear will make it easy for you to jump back to safety.

Some other methods can help you climb back up, assuming you have good timing, good reflexes, and the right things equipped: Featherfall will give you some extra seconds to live, granting a bigger time window for the above options. It can also give you enough time to open your inventory and grab gear you had not equipped. Don't be afraid to pause the game to locate the right item.
 * Double Jump or Triple Jump gear.
 * The Dash spell, assuming you have enough uses left.
 * The Grappling Hook can stick to walls or ceilings. Especially useful if you've somehow managed to fall off the Tower.
 * Grab Walls gear, but only in the Tower.

This also opens other possibilities: Special interactions
 * If you have a Scroll of Teleport in your backpack, now is the moment to use it.
 * If all else fails, remember that Wand of Chaos or Scrolls of Chaos can have the same effect.

Swoopers forced to fall off the level (through the use of Frost Nova, for example) will fly back up.