Awesome game, well done! I was really sucked into this game despite it not having any sound (I have an taste toward tower defense games so maybe because of that :D )
What I was really missing in the first run was a fast forward during the invasion part, but then I read the howto part again and it turns out it is there I just didn't notice it :D
After a while (like after 10+ rounds) it seems to become easier, maybe some nerfing of the resource drops needed, or making the upgrades more expensive.
Also extra points for having it done in pure c++ and having the game size under 6MB :)