When you go back to a place that you have already infected the population should be how you left it last time and not go back to how it originally was.
It would be nice to be able to pay a little extra money and get upgrades as soon as you build the tower when you need them fast but make it alot more than a normal upgrade would cost.
http://www.youtube.com/watch?v=dGnAcVZGAls
http://www.youtube.com/watch?v=dGnAcVZGAls
http://www.youtube.com/watch?v=dGnAcVZGAls
How to win with range, by me!