I agree to Melly, this is one of my favorite entries. As minimalistic as it is, the atmosphere is great, when you're able to always smoothly look around an see the whole sphere and the movement going on in there. That's especially cool when you are just entering the sphere and see the whole "arena" from outside.
I also think that the shooting is a very good addition, as it makes the game a bit more interactive than just indirectly placing the walls and hoping someone else drives into them. It just felt that at least shooting down the bots is rather easy, especially as the shots seem to live pretty long, so even by randomly shooting around it's possible to defeat someone.
I really hope you go on making this, it's already more than some interesting little experiment, but a pretty cool and rather unique game