Fun game, and easy to play! I liked it a lot! I played until I got the browser error.
For balance - early on, the rounds seemed too short. It felt like I was spending more time in the upgrade tree than playing the game.
Later in the game, I had to do a lot of panning around to see all the available upgrades. Maybe the nodes could be drawn a bit smaller? I kept instinctively trying to use the scroll wheel to zoom out, like in blender! ha ha
Also, I wasn't sure if the bat was doing anything, besides just flying around.