Great work! The mechanics work well, and the design looks great! I enjoyed the look :) Two little things you could change/add next are some sound effects and some background music (it felt so empty without it) and adapt the difficulty! I could place so many figures in the beginning so the enemies could come through... but happy to see more in the future! :)

Nice job, it's a good first level and a good basis to continue. It's classic, but classic works for a reason! 

The maths may need to be tweaked, the difficulty curve increases much slower than that of the currency earned. We quickly find ourselves with an army of Golems :D!