Fine, even if a bit limited
You should fix a couple of glitches:
-somehow, while upgrading an archer, my money got to NaN and since then I had infinite cash. I had already nearly ended the game but I'm sure it is not meant.
-the last "tutorial window" (the one which tells you to click on "next day") never disappeared and stayed there the whole time.
Also you should make it impossible to place pits in the grass or upon previously existent pits.
Again, the tutorial windows looked displaced to me, always a slot below what they meant.
You could add a boss, a once in a while, or different levels of building (ie: Town->City->Region->State->Whatever) or an opposing town or something else to bring this from its actual tower defence look to a RTS one.