Very simple, but loved it! As I always advice: if you add sounds and/or music, always give at least a mute button.
I noticed some sounds are from AoE. How about implementing the basics of that game? Houses for population cap, and farms as passive generators (either implicit or explicit by generating a coin). Then you could move the cost increment from the units onto the buildings. Additionally, since there's terrain acquisition, apply some of The Settlers' mechanics, where expansion is due to military buildings and/or pioneers. There's so much things you can do to the game...