Stamina system is really dodgy, when you run out it gets 'stuck' on 'out of stamina' for a while, before jumping back to full. Whether that's on purpose or not, it's no fun. Change it. Spell upgrade system also far from clear.
While the graphics are excellent by flash game standards, they're a little TOO high res, makes them a bit unclear. Giving some unit stats, and enabling zooming out, would be a help....but the gameplay in general needs overhauling. It's an RTS with no S right now.
The UI is horrible. Just because it looks flashy doesn't mean it's a good idea, kept accidentally placing things in the wrong place, and then unable to trade in. Also quality dropped to low for no apparent reason.
It is very similar to storm the house:woulda been a very decent game 5 years ago, but now it's samey & boring. It works, there's nothing majorly wrong with it....but there are many better games around.
On many towers, the upgrades do not display properly. Your first tower being -by far- the best, is a huge balance issue. I ~really~ like the monster graphics, but the path and house doesn't make ANY sense for the 'storyline'. And why is the information box half the screen? ya could easily make that smaller and have a bigger/better map. At later levels it also slows badly (down to 3-4FPS) at regular ~half second intervals...use some sort of random system to spread whatever check is doing that over each frame, rather than having all of the (towers/monsters/whatever it is) run that code at the same time.