I really love the game. The design is great and even after an hour the music isn't annoying. However I did find a flaw with the game. If you can get through the first few levels by building mills and such, then the game will go on forever. The fact that you can get 2 towers for every tower you spend and discards for peasants leads to having indefenitly many towers, which can be placed anywhere. And as far as I can see this leads to a game with no end. I am currently in level 42 and half the map is made of permanent buildings. The timer is over an hour and the towers do not diminish anymore, even if I use them.
I absolutely love this game but I think adding a level cap so the game can end would be a great idea. The game also seems to have problems going that high in level, it keeps stoping to load in the high levels. The level cap could stop this. Just a friendly suggestion though [which is totally not influenced by the fact that I want to beat the game and don't want to stop for hours cause I haven't found the end yet ;-) ].
So yeah the only criticism I have is the lack of and end. Everything else is great.