Air Defence 3!! A few suggestions.
Firstly, this is a great game and is the first to get into my favourites collection. It's simple, easy to get the hang of, fun and has good replay value.
I have a few suggestions though.
*Being able to put the units wherever you want. For example, just then I had 2 emps at the front of the line getting hit and hit, if I was able to put those two units and the back of the line safe from fire it would save me lots of money.
*More bullets controlled by the mouse. For example, maybe you could hire people to take residence in the tower and the mouse controls where they fire.
*For the 3 buttons down the bottom right that you can use when you get enough points from killing things, add one that costs maybe 3 points that heals all your units completely or makes them invincible temporarily.
*Greater diversity of upgrades, units and enemies.
*Maybe a "Fun" difficulty where you get super money for killing things and it doesn't get way harder :).
Great game, hope you can include some of those things in the next one. g00d d4y.