You've got a really fun gameplay loop here. I love being able to use scrap from the enemies to upgrade my buildings and for that to give actual gameplay benefits. I also really like the visual feedback of the building area changing as you upgrade it. Really conveys a sense on progression to the player.
I think the combat mechanics and enemies need a few more iterations to make them feel more dynamic, also maybe the camera zooms in a little to far during combat. I think a cool idea might also be to fold the building and fighting area into one giving and making the enemies attack the buildings as well. Though I realise that would also inflate the scape massively.
I had great fun playing this, I really hope you keep iterating on it, you've got a really good core loop