The placement is awkward at best. If you get a game over, you can't place any units the next time you play.
Also, you can place units off-screen too.

Looks like there is a bug. I Can't place more than one "tower".

Adding music, sound effects and more original graphics would help this stand out from the 500,000 other "castle defense" style games out there. Until then, not really much reason to play this if it doesn't stand out...