As others have said, it's a great game except for 1 flaw. It takes so long to get the upgrades towards the end of the game. There either needs to be endless challenges, or the wave number should be a factor in the money calculation earned per level.
Am I missing something? I feel like I have invested a decent chunk of time, and still have a sub par rate, no silver/gold rate, and see no way to increase the speed.
Not if its made by Apple.