The aim of the game is to remove all tiles.
You can remove a matching pair of tiles by clicking on the first to mark it red
and then on the second. Both tiles may not be blocked when you want to remove it.
A tile is free if the stack left or right of the tile is lower than the own.
The height of a stack of tiles is indicated by the gray border left and below each tile.
The thicker the border the higher the stack.