It's not often that a game can truly appeal to gamers of multiple skill levels. But I think that Medieval, the castle defense game from Brisk Mobile, is one of those games.
Set the difficulty to easy and it's challenging but winnable, a good incentive for casual gamers to keep playing. Increase the difficulty and it requires a whole new set of strategy and skill. Medieval has three difficulty settings, 6 troop styles which include unique war machines, and 8 different weapons all with additional upgrades. This combination of settings, weapons and difficulty ensures that Medieval has enough tricks to keep serious gamers strategizing for quite some time.