This is a great game and has been said by others it's a good place to build from too. It took me a few moments to get the hang of where I could place certain buildings etc. Others have already mentioned the idea of upgradable buildings and more enemy variety, which I agree would also be great. Something I'd suggest is being able to see how far away buildings have to be from another of the same type, as that is something that I had a bit of an issue with.
All in all it's very fun, well done :)