I haven't experienced any crashes. Played it for a few days, and progressed to the end of the first zone. I can't understand how some enjoyed this boring excuse for a money drain enough to rate it higher than 2 stars.
First, just the setup is boring. Pick you monsters and pull a handle to have the game select the invading force. Select your defender in rock-paper-scissors fashion and every now and then tap the screen to execute the monster's special attack. Your monsters do not gain experience. Instead, you get them stronger by spending the gold you earn from repelling the slot machine results.
Naturally, there's iap in the form of the gems you can use to summon stronger monsters than you can with gold. Why wouldn't there be? In the end this game fits the cookie-cutter mold existing currently where a boring, extremely repetitive game is made that increases quickly in difficulty so you CAN progress without spending money or filling out surveys or signing up for other things to get the developers money, but you will progress so slowly the game will become a chore. And for a game that's not very interesting in the first place, I can't see why anyone enjoyed this enough to give it some of the 3+ star ratings I've seen.