I'm on my first rewatch, I feel like such a dork I didn't notice this the first time. This is 1000 years before Himmel and co, and all the buildings and dress have a distinctly Mediterranean, ancient flavor. It looks like Rome, or Ancient Greece.
1000 years later, things are distinctly medieval.
This was such a cool touch, and I completely glossed over it at first. Frieren is even still wearing ancient garb when Himmel seeks her out.
It's all the little details with this show, really draws you in.