The must-have Monument Valley is currently free on iPhone and iPad
Twist of fate
The stunning and essential iOS puzzler Monument Valley is available for free, right now, on the App Store.
The game has you manipulating impossible architecture, to guide Princess Ida through a series of Escher-esque levels. It was one of our favourite games of 2014, and we called it a "brilliant, breathtaking, and brief adventure".
If you're on Android, you can get the game for free via Amazon Underground.