I love the concept, you really deeply integrated the shape limitation into the gameplay which is very cool, and it works really well - it is a unique twist on the arcade shooter.
One of the main ways you can increase the "fun" in a game is to focus on the decisions the player has to make. In this game, I don't really feel like I make many decisions at all. I would've liked to see different enemy types - that is one of the more fun decisions in a game like this! Smaller/larger circles with different health, a filled circle that moves in a different way, that way I have to prioritize what I'm shooting at.
Your colours could also do with a little bit of work I think - they are very saturated, which hurts my eyes a little bit after a long time, plus it's a little difficult to see the blue on the black background.