Hmm It's a big flaw that so many people don't realise what it's for, given it's the main focus of the game!
The player can create gems which amplify the abilities of a tower and grow over time, gaining new abilities.
Viewing post in Island of 1000 Crabs jam comments
Okay, had to go back and play again, that just makes it even better. Also realized I could control where the towers aim and add upgrades this time, loads of fun. Got this error after one of the waves http://imgur.com/a/kLvt8. Great stuff