That was a very fun tower defense in Pico8! Very cute sprites and groovy chiptunes! I managed to cheese my way through it though by not killing the slow moving enemies through exhausting ammo, and mass building money making towers in the meantime. I guess it mixes clickers with tower defense? Doesn't seem too meta to me, but what was there was polished and well done!
Note that placing towers near the top of the screen obscures the upgrade menu.