cheaper towers would help and cheaper level upgrades for the creeps

I haven't actually got a game in yet, but I think it's best we get plenty of people playing before tweaking.

ya, I agree.... I know for sure the towers will need some tweaking. It was very hard for me to try and come up with good numbers (for their damage, costs, etc) when playing and testing a multiplayer game by myself. I tried to come up with numbers that might be close.... and after it's live and enough people have tried it we can work on changing it to make it feel right.

I would make it more expensive after each level.
if you upgrade the first level you should already have in income of around 1400.
the next upgrades could be more expensive, so you got the choice to deal with the actual ones or spare the money for an upgrade, which means you are more vulnerable while you spare the money, (meanwhile your enemy will increase his income, maybe he decides to do the same, maybe not, ...) but you'll be more "deadly" after you reached it.
would become an interesting part of the strategie you play.