Defense grid looks the one thanks guys
Good choice! There are some levels that I just couldn't crack without 'ahem' the assistance of a cash cheat!
Those are very, very rare however and probably for a hardcore tower-defence player, rather a good challenge.
It's a really rewarding game though when your strategy clearly works onscreen. The one piece of advice I would say is to make sure you have anti-air covered. No air unit is hard to take down in this game, but they do give you plenty of cash when brought down.
Another game m8 (also on Steam) is Sol Survivor. It's not as polished as Defense Grid imo, but it's a good alternative. They do a demo of it as well, if you want to try before you buy...
http://store.steampowered.com/app/45000/
D.