The style is minimalistic but effective!
Since there isn't some tutorial or any explanation it took a few minutes to understand better the game.
I think that the small key explanation isn't right away readable when you are in the opposite side of the planet, also it would gain to be a litle bit bigger.
We had a similar idea for taking houses/buildings , that's quite funny ^^ I really enjoyed seeing how you took advantage of that idea in your gameplay (changing the color of the houses when in an other territory, protecting the city by moving them)
The sound of the bullets firing is a bit repetitiv, maybe it would be nice to change randomly the pitch a litle bit, or any other idea that could make it more confortable in a longer session.
I must point out that the controls were quick and simple to understand.
Ps : I like the movement of the slow move of the stars on the background , it adds some nice depth !