I really liked this game, it has so many good features and, in my opinion, it is very good.
The game is easy, has nice graphics and a funny story. The difficulty increases as you progress, which is good. Also, the battlefield's simplistic design is just beautiful! I also like the boat/springs idea, very creative. This game also obligates people to plan their moves (especially when 'Vikings' start to spawn), because one doesn't wants to become trapped between two hordes. In this case, you can use the stomp hit, which I think was a very good implementation to the game, making it easier.
About the music, I just love it! It never gets too repetitive and fits pretty well. Great job with this, it really looks like you've put a lot of effort into this and I hope you make a sequel!