Crazy Defense Heroes, page-6

"This app is still in development. Be one of the first to try it and provide feedback"

Also when I tried to download it on a friend's device it said the had reached the max users at the moment. Word from the big man Robby Yung is that they're doing a phased release schedule and will open it to the public when they feel comfortable it is stable and almost bug-free (nothing is ever bug-free)