This is by far my favorite iOS game franchise and in my opinion the best TD franchise ever. However, there are two missing features that drive me crazy.
1. No ability to save progress. iCloud saving ability would be ideal, but it seems that you should at least be able export basic information about progress to a CSV file for backup and restore purposes.
2. These levels take a long time. A lot of people will not be able to finish some levels in one sitting. They'll get interrupted and have to switch out of the app or put the iPad down and come back later. Sometimes when I come back to KR (any version) it works and I can resume. Other times, I come back and the main menu loads!! After spending 30 minute playing a level, I have to start all over again!
Please fix at least the second issue.