It's an ok defense game
I can see why your score isn't higher and i can give yea some tips to make it better.
First and foremost those arrows that shoot at you constantly shouldn't hurt you while they're in the air. They make the game waaaay more harder than it should be.
Secondly, it was a good idea to add in a button that lets you go instantly to the buy menu and it works for some games, but for this one i think it'd add difficulty to the game if you made the player wait untill a set time.
Thirdly, I didn't get very far in this game namely for the fact that your fast low healthed units completely ripped through my castle. Personaly i think that if your going to have units that move that fast, your gonna have to reduce they're dmg. Even if they are just suicide bombers you should have they're attack some where in between your foot soldiers and your golems.
Lastly, I would check out the game again. I think there are a few glitches still. I was trying all the buttons out to see what they and some how i managed to get the game to go back to the title screen and it wouldn't let me play till i closed the window and reopened it. Also your preloader didn't work for me, but it might of just been me
but you might want to check that out.
This game has definite potential. It just needs a few minor tweaks and it'll be definite front page material. But as it is right now your just short. So Keep with it man