The Galleria has been mentioned in the short stories but never, until now, seen in the comic.

The "rain" is actually ambient moisture that rises to the ceiling and condenses (the ceiling is cold, because it's nothing but frozen airless moon above) and then drips back down. It doesn't rain hard, or continually; it's more like an occasional drizzle. Sometimes there's fog as well.

The bridges are inspired by the bridges in large towns of late medieval and Renaissance-era Europe, which were lined with shops. Behind these shops, a network of makeshift boardwalks, ladders and so forth provides private access between the businesses, as well as a place to store extra stock, grow backyard gardens, stash inconvenient children to keep them out from underfoot and so forth.

The Galleria in my head is even more chaotic and crowded and built up around the edges than I managed to convey on these pages (there's stuff built up the walls, and dangling from ropes, and so forth) but hopefully this is close enough to give you an idea of what it's like.

Also, the Galleria Merchants' Association -- the cartel that runs the place -- keeps a lid on some of the more excessive flourishes that entrepreneurs might otherwise be prone to. Shop owners pay high protection fees in the form of space rent, and the rules are fairly strict (downright draconian by Kismet standards) but in return they get a place that's largely crime-free, with minimal panhandlers, tastefully discreet prostitution, and a considerably reduced chance of the shop next door accidentally setting everything on fire. Plus, of course, retail space in Kismet's #1 shopping district. If you don't want to deal with the rules, you can always put your flaming-hog-racing establishment in some other part of the tunnels.