I have good experience on tower defence games so I know some "must do" tricks that usually fit in most tower defence games. Mainly fill up all tight corners that covers the most areas and then upgrade the hell out of them!
If I may suggest however for future releases; I would make the Upgrade system more streamlined. Like they all have 3x upgrade levels, Or at least show in the UI how far along they have to upgrade fully (like a star system that has 5 dim lit stars and each one lights up signifying the upgrade system).
And I would diversify/increase the reusability of older units, cuz when points started coming in more frequent, the Military guy was a clear upgrade from the normal guy by all means.
But all in all, well done mate , I honestly enjoyed the experience! ^^ And Glad to hear you enjoyed mine as well! :D