Twenty is a colorful arcade game for iOS and Android that starts at 5, looks easy until 10, gets fiendish at 15 and challenges you to get to 20.
Initially it's a simple number matching game. Combine two matching tiles to create a new tile with the next higher value. Your score is simply the highest value piece on the board.
When linked-tiles are introduced (starting from 10), then the game reveal its true colors. Linked tiles must be moved together, and this creates restrictions on how tiles can be matched or whether a tile can be moved at all. Fortunately, links are broken if one of the neighbouring tiles participates in a match. This mechanic is totally unique and is where the real challenge of the game lies.
The free app includes the standard single player mode and also a two player mode, in which you and a friend race to get to Twenty, trying to steal tiles from your opponent's board while guarding your own.
An in-app purchase also unlocks a number of interesting variations, such as Zen, Bubbles, Drop, and Flip-flop. A 3 year old child can play Zen mode, but Panic mode will have the most hardened player tearing their hair out.