Promising, both in itself and for its implications for how prepared they are to repel high-magic infiltration. There's only one floor above ground, so unless they can spot a likely-looking prisoner through any of the openings they start with the one farthest from the door.

Then they can drop into dark hallway. Through the dim light visible in the windows, it looks like it terminates in a dead end off to their right, as though someone liked the idea of ominous corridors more than the substance.

Nenio mutters something under her breath.

"It's not a secret door either," she whispers. "I just checked"

That is very cool and Brenda will want to hear more about door-finding later. How about the other direction, does that end go anywhere? Are there cells in this area or just blank walls?

Once they round the corner, it feeds back into the main hallway about a dozen feet past the entrance. There are a few other branching hallways like it, but the main path is obvious both from being a straight shot inwards and having actual torches providing light to see by.

Brenda has no need for light, but the torches are useful information nonetheless. They'll want to sweep this whole level before they go to the basement, unless they've found both their rescue-targets by then, but might as well check the main corridor first.

The main corridor leads directly to a stairwell, the downwards steps similarly lit to the passage leading up to it. It's hard to be exact with proprioception, but judging by the distance traveled they've got to be pretty close to the opposite side of the building from the entrance.

Off into the branching hallways, then, going systematically clockwise and keeping an eye out for any doors. Brenda's sense of space is good enough with the headband that she'll be able to tell if any chunks of the floorplan are inaccessible.

Most of the corridors are similarly empty, but a diligent search will reveal a locked door leading to some depressing living quarters and an extremely doompunk armory. The latter has an impractically large number of glaives and scythes, but some concession was made to practicality in favor of spears, daggers, and scimitars. The hallways continue to come up blank on hidden doors, but there are a few suspiciously large spaces that might correspond to something or might just be randomly solid stone all the way through.

They're not here to rob the place; for one thing, they need the bag of holding empty to use as the getaway vehicle. Brenda very quietly points out the suspicious areas, but if nobody else has a quick way into them it's worth going down to the first level of basement first.

Ramien can try a dimensional door for one of them, but if there's nothing inside he'll be forced painfully out of the rock and he doesn't have a good way to leave again after if there is space inside. He doubts they're hiding the prisoners in a secret passage anyway; that would make it hard to bring them in and out.

At the bottom of the staircase, the first cells are visible; judging by the construction of the bars and locks, this is a low security area, and the only prisoner inside is a solitary Dretch in the third cell on the right.

Brenda is pretty sure that isn't Arueshalae, but is now realising that she should have set up some way for Arueshalae to identify herself in a more ongoing, passive way, and/or accepted the social awkwardness of asking what species she was. Regardless, they should find both of the people they need to rescue before doing anything obtrusive that might put them on a tighter clock. She observes the dretch silently and wafts onward.

The medium security cells are more thoroughly sealed, with small reinforced glass panel on each cell the only way to see in or out. There are a handful more demons here; mostly schir, but there's also a Brimorak and and Incubus. The entire section has another locked gate to get entrance to, but the designers didn't seem to have considered gaseous form as a security vulnerability.

"Blocking line of effect for summons, most likely - see how the air interchange doesn't allow an unbroken line through anywhere? I'm surprised to see something so sophisticated in good upkeep, despite how little use it's getting; someone important among the demons must have this place as their pet project."

"Yeah. And I screwed up--I don't know how to identify Arueshalae without revealing we're here to anyone else. If we find the crusader and they don't know her, I might need to sneak out, do another Sending, and sneak back in. Sorry."

"I prepared detect the faithful this morning, so I can check any likely candidates, but I'll lose it if I have to cast any other spells after starting."

Past the middle security wing is another flight of stairs leading down, terminating in another hallway of cells. Unlike the previous floor, none of these cells block line of sight, but they do look more secure than the first floor.

"Ooh, a trap!"

Brenda freezes* and looks around for the trap. "Where?"

*Metaphorically. Chemically, she doesn't even condense.

"Right in front of us! Look with detect magic, not your eyes."

It's impossible to see Nenio's still invisible expression, but the dissapointment is clear in their voice a moment later when they continue.

"It looks like it's just an alarm and a dimensional lock. That's disappointing, there's not even a puzzle."

Vaporous squinting. "I see it but I couldn't've identified it. Do you know how to disable it without setting it off?"

"If it's an audible alarm, I could use silence, but that doesn't do anything about mental ones. Can you tell which it is?"

"Abjuration isn't really my specialty, but it looks like it's audible."

Ramien mutters a quiet prayer to Desna, and holds up his butterfly necklace, after which bubble of illusion magic wraps around the center of the alarm.

"Let's move. That should make it so nobody hears any of us crossing, but only if we do so in the next minute."