Starting a game:
1.Determine randomly who will go first. Each player starts with 20 life.
2.Deal three cards to each player.
3.Starting with the first player, each player may discard any number of cards and redraw that many. Once all players have done this, shuffle the discarded cards and put them on the bottom of the deck.
4.Put the top seven cards of the deck face up in the middle of the table. The deck becomes a library that's shared by all players.

Playing the game:
1.When playing Wizards' Tower, your draw step gets a bit more interesting:
-First, choose one of the face-up cards and add it to your hand. If there aren't any, put the top seven cards of the tower face up in the middle of the table, and then choose one.
-Draw the top card of the tower.
2.All players share the same library and the same graveyard. If you cast a spell that targets a card in an opponent's graveyard or manipulates the top card of your library, those cards affect the shared library or graveyard.
3.The face-up cards aren't considered to be in any game zone.

Other than that, the game is played as a free-for-all Magic game. The last player left standing wins!

Source: (https://magic.wizards.com/en/articles/archive/making-magic/wizards-tower-magic-format-2013-07-23)