17. because of later pieces joining together in 3's, it seems it might be impossible to reach 20? (do you ensure there's a soln?)
(android version) i found the touch becoming unresponsive on the edges, in later stages. pr because i'm getting frantic, and using a crappy phone. but if this could be improved (eg larger hit area on the edges - or zoom to cover whole screen), it would be worth it.
apart from the feeling of frustration, the gameplay problem is you lose much time repeating the movement x2 or x3 times, which is crucial to avoid locking joined pieces and large surface area towards the end.
PS would be better if it didn't require any permissions at al. it just seems to be storing twenty.jpg - necessary.
also "throwing" blocks with momentum as another comment said would be cool, and wouldn't undermine gameplay.