So there's a recent game app of Shingeki no Kyojin in the Japanese App Store. The game mechanic is very simple: your goal is just hit the targets (on the Titans' necks) with your finger or fingers (depending on your tapping style) and the progress will go to a faster pace until you die.
I guess I'll admit that it can be fun for a while and it doesn't take much memory either. However the advertisements can get quite annoying since it's a free game entirely (especially on the menu). Also there's no other bonuses (except health packs) or upgrades. Basically it's a flash game or a mini game you would usually see but due to it being based on Shingeki no Kyojin, it's an okay game for fans of the series.
Conclusion: Simple goal and can be fun for a while.
Note: Only available in the Japanese App Store.