Great Game
This is a great Tower Defense, has a lot of possibilities to play with different strategies.
The way towers upgrade themselves based on experience and level and the way the castle levels up and provides AU (gold) is just clever, then we can focus on strategy and not so much on constantly checking towers to upgrade them.
The support building system (farm, mine, training, etc) just add the twist to the strategy planning the gamers need.
Overall difficulty slightly above average of regular Tower Defense games.
Two things I would improve: adding a grid system for placing down buildings, it can be a little tricky at times to fit all you want; and adding medals (Newgrounds). Possibly also adding a "Start Next Wave" button.