Good, with a bit of work this could be awesome
As people have mentioned, their needs to be more this game than just peon slaying and sending out your troops after they die.
I think you should have it so that you can send multiples of the same troops, as this would drastically increase the stratergy. You should have some units that are strong and weak agisant certain troops, and troops that work better in diffrent situations. Wizards are way too expensive, and dwarfs and Knights of Rohan are way overpowered. As others have suggested, I think the hero should have a health bar and be allowed to fight,but as an intresting element you should keep the Peons in so you have to choose between protecting the line (while getting cash) or helping your buds.
It was nice how you made the effort to include a variety of missons, but the game still got a little monatous at times. A password/save feature would be good. I feel their should be an option to minmize the text box, as on the 3rd map, i would often hear fighting and not be able to see what was going on.
The bosses added a nice variety to the game (particurly the spellcaster and the demon) but I thinkthose missons were flawed, because you'd just make a huge gangbeat to take out the boss, and you'd either die, or kill the boss and get so much cash that you just flatten all the enemys on that level into the dirt (with the exception of level 6(my favourite), but you still got way too much cash).
Overall, nice game, if you make a sequal and fix these faults it will be one of the greatest games on Newgrounds