In Cubulus you want to be the first player to create a square in your color on one side of the cube. Do this, and you win the game.
On a turn, you add a colored ball to the 3x3x3 playing area by pushing it into place – possibly pushing other balls out of the way – or change the order of balls in a completed row by pushing them one space, then reinserting the ball pushed out of the row. With two players, one color of balls is neutral. Keep your eyes on all sides of the cube to prevent others from squaring off for the win.