This is a cube for the judge's tower format:

Rules:

All players share a deck and a graveyard of the 300 cards in the cube. You own every card you draw for purposes of things like Homeward Path.
Begin the game with 3 cards in hand.
You must play every card in your hand as soon as legally possible. All optional modes are mandatory, to the extent that you can fulfill them. (Must say yes to all may effects you can)
You must activate every activated ability of permanents you control once per turn per legal target as soon as legally possible. Always activate abilities from the bottom of the card up.
If a spell or ability has X in its mana cost, X is always 3.
You must attack with all legal attackers and Block with all legal blockers whenever the option is presented to you.
All players have 20 life that resets after every turn.
All players have infinite mana.
You lose the round if you commit a game rules violation.
Play the game until there’s a winner, that winner gets a point, then exile all permanents, all cards in the graveyard, and all cards in hands and start the next round. When the deck is empty, the player with the most points wins!