Interesting concept, and one I've been thinking of prototyping for years myself! Glad you've done it so I don't have to...
Well presented, love all the details like the animated menus etc. It's not super clear what's happening and what you can do. Needs a couple of hints to onboard you a little better. I only figured out I could spawn new units by clicking in the edge after I'd already lost twice!
I'd have also appreciated indicators showing what the different units did, how much HP they each had, how upgraded the each were etc. Likewise with the single hero - how much health does he have? How much damage does he do? Explaning these things to the player in a clear and concise way would go a huge way to making your game feel better.
As it was, I played about 5 times, and in the end could hold the hero off indefinitely. I never new if it was possible to defeat him though. I could see my "money" clicking up at a decent rate, and spawned in a few of the red guys. They died quickly just like the cannon fodder yellow guys. I had expected them to behave differently, like maybe have a longer ranged attack or something.
Still, well done for getting a totally working game done in 48 hours. Not an easy feat at all!