This feels like a great game! It's nicely done, and pretty damn polished.
The pacing feels just about right, and the different sound effects for the different upgrades makes each one feel like an achievement. It's simple and effective; there doesn't feel like there needs to be much more than this, other than perhaps a few tweaks.
The sound effect when generating gold is a nice touch but gets annoying quickly, it would be nice to have upgrades that multiply the 'generate gold' counter, and it would be nice to have different coloured buildings for variety.
Other than that, very nice job! Well done.