This takes me back, it has been a while since I last played a tower defense game. The only complaint I would have is, that sometime the projectiles seem to be too fast and just zoom past the enemy and fly around them for a while before they actually hit. But that's a minor thing I like the variety in towers you had and the wall placement mechanic made a lot of sense and worked well.
I didn't get to a lot of the mosters from the screenshots, but it looks like there is a lot more to this game than I saw, so I might have to come back later. Really well done!