Love the game. It's pretty well rounded. If I could make a suggestion, it'd be a tally for how much gold I make all together from the buildings, and another tally for how much I make from all the countries I've conquered.
I'd like to see some upgrades for the multishot and the double power drops....perhaps more damage on the multishot, and longer duration for the double power. I've also noticed if you save your upgrades for the armor and health, until you're in battle and almost dead, they refill the respective bars.