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[twillight]

It is an android game. Because those exist too.
And it's entirely free.
Sure, there is the possibility of micro-transaction, but you never have to. It takes a while to upgrade yourself to be able to beat all the levels, especialy on the harder difficulty, but sure, it is possible, and isn't a chore. Also it is a convenience issue if you want to play whenever you want, and not wait until the energy bars fill up.

In addition it has several minigame-playmodes to spice things up and offer you stuff to do if you are full. My favourite is, where it rains weapons. You can't use your regular storage those times, but you finaly can reliably use such things as the A-bomb, which clears the screen, but you can only pack 1. On the "it rains guns" levels you can get 3-5 of those, what is hilarious.
It is a very well designed game, 5/5.