Interesting premise! I'm always a fan of a 2d shooter in a jam; feels nice to just blow some stuff up. Creating and defending your nodes gives the player a nice objective other than just "kill all the things", too.
The aiming and shooting controls feel pretty good. I'm glad that you made the gun auto-fire. Since there's no disadvantage to just holding down the mouse button the entire time, I wonder if it might make sense to just have the gun fire by itself the entire time and just let the player aim?
I think that the only thing I'm really missing is an ending; a score doesn't really tell me a lot if the game just keeps going forever. Something as simple as a timer that ends the game after 60 seconds/etc. could work well here.
Nice submission!