Thank you so much for playing the game and finding the time to provide the feedback! Honestly I didn't think anyone would even go all the way to the last level, so I didn't put any heart into it. Since you seem to have enjoyed it so much, I will work on it more and clean it up (it's a little embarrassing that I didn't notice the floating building tbh). Also, fair point about the units going through non selected buildings, that is something I'll clean up.
I think giving the player the chance to see the level before starting is a great idea. I am not sure what you mean about customization though, maybe something like letting the player choose the percentage of swordsman/archer/supply bases from the map after seeing the level itself?
You definitely have an eye for detail about the upgrade also ending up selecting the bases. I think it has something to do with raycast from camera going through the upgrade button and selecting the base - it's a good observation and I will try to figure out how to fix that.
Thanks again for the detailed feedback, it was really valuable. I do feel encouraged and I will keep working on it!