You remove cubes from the board by forming "chains" of three or more adjacent cubes of the same colour. Diagonals do not count. That is, to form a chain, cubes must be touching face-to-face, not edge-to-edge or corner-to-corner. The main object of the game is to clear cubes from the three levels as fast as you can by forming chains of three or more cubes of the same color. A game of Cubis consist of three levels. Each level begins with a number of cubes already arranged on the board. You add additional cubes to the level, one by one, as you play.

You can choose any of the seven rows or columns in which to add the next cube. Cubes can only be added from the two front sides of the board, not from the rear two sides. Added cubes slide along their row or column until reaching a cube that's already on the board, or until reaching the far end of the board. When you form a chain, points are scored and the cubes that make up the chain are removed from the board. Cubes above them then drop down to fill in the unoccupied spaces. When this happens, additional chains can be formed. A second or subsequent chain formed on the same turn is called a "Cubis" and gives you bonus points.