Awesome
this qualifies as one of the best tank games I've played.. just a few things need updating:
a box that shows you an explaination of the upgrades, such as "option node" you dont know what it is untill you try..
the background of the land.. it looks like a default background template from office 2000.
the upgraded weapons should look a bit meaner, different colors and such.
The choice of enemies isn't that big.. perhaps some fixed base to attack would be fun? just spraying ideas here.
maybe more choices of tanks, so you look as badass as the others :P
(once I got stuck in a tree as well...)
but what you have here is already very fun to play, and is an example on how it should be done! :)