Excellent game, a little... er?
I liked the concept, although I found the game itself a little small. To improve the game I think you should perhaps, add some more levels. I found that the game was over by the time that I'd got my infantry to level 2/almost 3. If you make improvements to this game these would be the ones I'd suggest:
1. Make the game longer, add some more levels with higher HP castle to fight against
2. Make a 'skip word' button, perhaps so that you lose 20 cash, I found one word I couldn't finish (me being dumb)
3. The units (infantry) are good, perhaps add 1 or 2 more of them
4. Add some sort of storyline? That's a little random but I'd think it would be a little more fun if there was a purpose to it, and if you do add one - give a skip button for people who can't be bothered reading it :p
Other than those, it's a great game - and a great hybrid of defending a castle and hangman. Keep up the good work :)