Most of the cities you have to target are in France and Germany. For no good reason whatsoever. When there are thousands of cities to choose from across the planet, why so few 'hot spots'? I'm lowering my 5-star to 3.
Interesting, but since I haven't any way of searching for some of the obscure symbols (level 17 is one of them) I don't see how this is supposed to be fun.
The music really helps and I love the black-on-white look; however, it does get a bit monotonous. Maybe if the strategy were a bit more intriguing I could stay with it. Still, excellent work.