I think the story was amazing, however, the difficulty curve was that all the game was pretty easy, the last level was a little more challenging, and all of that was fine, it was not a game that needed to be extended. My problem is with the final boss. It is, unlike the rest of the game, completely unpredictable. It is too hard when the rest of the game was doing fine in difficulty.
It killed my experience, since i didnt watched the ending but probably will do it another day.
I would recomend to put a difficulty option. Not too easier, but that it would put more space between attack and attack waves