Great Concept/Typing Engine
I enjoy typing games and defense games and you were able to successfully merge the two genres together. You definitely have a good start and know what you are doing so I will focus this review on areas for improvement. In regard to the defense aspect, I would implement levels with upgrades etc. ala other defense games.
The primarily flaw with the game, as mentioned by other reviews, was how keystrokes overlapped between words on screen. This was an interesting aspect when the difficulty was low, but takes away from the pure typing aspect of the game.
The graphics were virtually non existent and should be introduced/improved for your next version.
I found the game rather easy but I am sure that others found it quite hard; therefore, I suggest introducing difficulty levels or a difficulty slider.
Lastly, I experienced some lag/slow down as the game progressed. I am not sure if you can improve upon that but it is something that I would look into.
Overall, you have a great engine that I think you can successfully build upon going forward.