Pretty tough
It's pretty tough. You ear money at a very slow pace and a lot of the enemies are unstoppable unless you unlock the nuke and use that. Only problem is, I could only get the nuke on easy :( Medium was hard, and I lost pretty quickly.
It was also kind of a pain that there wasn't a tutorial, so I ended up taking a few hits from air plains due to the fact that I didn't know there were tabs containing AA guns.
It's actually pretty good, but it starts off rather slow and is focused for the hard core people. An overall good defense game.