A good start
But it needs work to make it into one of the best games out there. Good to see that you got the NG API working for this, so you've got the ability to get these things sorted:
1) Try zooming in a little more when you come to draw the images - you'll get better control with the mouse or tablet and it will mean that you get a massive boost to the look of the project overall. Perhaps use a smaller tool to draw with as well.
2) Try to make the path that the monsters take look a little more than just straight lines. If this is a fantasy setting, they probably haven't got things like lawn mowers to keep the grass verges tidy :P There was a track around the level that the monsters couldn't walk and as a result, I placed towers in the wrong places. Perhaps institute a grid system for placing things like towers. I'd look at Gemcraft for that, by the way.
3) Sound would always be a plus on this piece - from a little jingle on the main menu to the sounds of things getting destroyed by the towers themselves and even the towers firing.
4) Why not make the towers stationary and have a turret positioned on top of them? It would make the animation look a lot better, as it's how something mounted on a tower would fire, after all.
5) Where is the life counter for the level? I cannot see how much life I have and how the monsters getting through to me are affecting my chances of winning...
6) This is the advanced one - why not try instituting a level system? This was the main gripe in this game for me - it got a little easy after a while and I was levelling the towers up to keep up with the enemies, as they came along. If we got to the end of the level, it would certainly give me something else to focus on.
[Review Request Club]