Awesome game overall, I really enjoyed it! However, after around round 20, it became quite straightforward to win. I completed round 40 just to see full upgrades but I would increase the difficulty for later round (harder enemies, more entry points etc.) Also, reallocating buildings is quite overpower, I would add time delay for functioning again and/or have some cost. Upgrading multiple times is inconvenient as reselecting the given building is required at the moment.