Also called Mahjong Solitaire, this is tile-based game where you have to remove all the tiles from the board by matching two identical free tiles.
This awesome Mahjong game was created by ffalt and it is licensed under the MIT License.
The game difficult in this game is influenced by two variables: board formation and tiles style.
The number of free pieces is directly affect by the board layout, so less free tiles (at any moment in the game) implies in a harder game. Here are some examples of boards formations grouped by difficult:
Playing with traditional tiles also makes the game more challenging. Take a look at these tiles sets grouped by difficult: