It's the U-Boot-Bunker in Brest!!! https://de.wikipedia.org/wiki/U-Boot-Bunker#/media/Datei:Concrete_U-boat_pens_at_Brest.jpg (That's why Ro and Luigi are so comfortable.)

Bunker Hotel Catacombes

Is that a reference to the French movie Bunker Palace Hotel by Enki Bilal? Or is that too far-fetched?

At Bordeaux there is something similar to this day. It was adjacent to the Italian base, but it was not used by them, only by U-boote.

Historynerd said:

At Bordeaux there is something similar to this day. It was adjacent to the Italian base, but it was not used by them, only by U-boote.

The cat-ear like structures on the corners seem to be only present on the Brest bunker. Also Brest seeming to be a center of action in the current event (see ido f.e.) it seems fitting. (Speaking of ido - this strip has a "ido-ish" touch to it.)

sanitaeter said:

The cat-ear like structures on the corners seem to be only present on the Brest bunker.

And both Luigi Torelli and U-511 knew that facility very well.

https://www.battlefieldsww2.com/brest-u-boat-bunker.html said:

The bunker at Brest is still in use today by the French Navy.

Uhhhh... hmmmm... using that bunker as a hotel might be a bit problematic.

Unless it's less "hotel" and more "barracks".