I think it would be a good idea to have some sort of optional account system for kingdom rush. For example, when you start the game you can choose to either create an account, login, or dont make an account. An account would be useful for storing your game saves and purchased items.
For example, if i have an account, and lets say my username is 13mastercool, i play kingdom rush on my iPhone. Then, when i go to my iPad, i login into my account on the kingdom rush app, and then my same save from kingdom rush on iphone will be on the ipad, as well as any purchases. As well as when i go on an android or a pc to play, i just have to login to my account on the game and then my save will be there. I think this would be awesome to have, and it could be optional whether to use an account or not.