Interesting game, but there is a problem.
You can just keep making circle close to the tower around it, and you are immortal xD
You should add an area around the tower where you can't draw maybe or think about another mechanics to avoid that :P
Maybe like a red zone around your tower where it cost you 1.5X or 2X more ink, so you can always defend until the last second before getting hit but it's better to destroy bullet further away because it cost less ink.