If pushed in that order, the stones do allow themselves to be depressed into the floor. There's a loud creak from somewhere but the octagonal bit of rock doesn't actually move. 

"Huh. Could be stuck? It's really old." 

"I could Dimension Door down but we're trying to be really careful of strange magic..."

"I could give it a shove." He does so with Fetching instead of his hands, in case it tries to bite back or something. 

With another groan, the section of stone drops lower into the floor. 

"I think it's just really old. Maybe if we pour water here again it'll lubricate it a bit..." 

"Or rust it." He tries to see if he can get a view of the mechanism. "I guess if it does rust it'll be after we leave, so worth a try."

One side of the shape has sunk deeper than the other, and has maybe slid a bit sideways into some sort of slot under the floor; the crack on the opposite side is a few millimetres wider. 

Maybe they can try grease instead of water?

Sure, if their party is sufficiently prepared that they have grease on them too, that does seem like a better idea. 

Of course they have grease on them. They can try to get it into the door mechanism.

Vanyel can help shove it in there with some detailed Fetching pushes. 

Greased, the stone slides neatly out of the way. 

There are more stairs. Very narrow ones. They appear to go quite far down. 

"Mind if I put a spot of paint in this place so if we come back to it later we know it's the same place?"

"- Wait, what? Why wouldn't it be? Er, sure, if you want to." 

"In case it's the kind of dungeon that changes around as you move through it," he says, adding a red dot on the ground. "Most aren't, but it takes a minute and saves you ten hours sometimes."

"Is that a thing in your world? It sounds really hard with our kind of magic, although," brief exhalation, "I'm hesitant to put anything past Urtho, I don't have the faintest idea how the floating spell works or how it's still active at all." 

Vanyel sends a mage-light down the stairs ahead of them. 

"The sort of wizard who'd built sixteen superweapons for fun would in our world absolutely build a tower that changes around behind you but it does seem like the kind of thing that's easier with our magic." And they can descend the stairs.

At the bottom of the stairs is another big, round room, only slightly narrower smaller than the vault above though with a much lower ceiling, but this one, rather than being bare and minimalist, feels downright crowded, workbenches and shelves everywhere, all of them absolutely covered with stuff. Glassware and jars, still intact, showing dust and residue where chemicals had once been. A table dedicated to wood-working, with a lathe, clamps, vice, and assorted tools. Another station for metalworking, and one for glassblowing, with several beakers and more complex apparatuses in various states of completion.

The shelves are loaded with incomplete projects, scattered with notes and diagrams. A fine sheen of dust covers everything, but surprisingly little. (There haven't been any humans around to shed dead skin cells in millennia, after all.) 

Here too, the air is perfectly fresh and breathable when Vanyel checks. 

"Huh." Anything magic?

Most of it is! Almost none of it is recognizable, even to Vanyel. 

"This looks kind of dangerous to touch. Not that I'm not tempted."

"If we go slowly I can at least tell you if things are going to explode. ...You can touch that brooch, if you want, it's not magical at all." 

It's a beautiful piece, half-completed, in the shape of a hummingbird; the inlaid mosaic of tiny agate-pieces as feathers is mostly unfinished. 

"How did he have the time," Mahdi marvels.

"Maybe he was one of those bizarre people who only need four hours of sleep a night, I know a Herald like that. And - I figure he didn't have time, after the war."

He leans in to try to read Urtho's notes, and is stymied by the fact that it's both in an extremely archaic form of a language he barely knows how to read in the first place (Shin'a'in and Tayledras use a different script from the Valdemaran one, ideographic rather than a phonetic alphabet), and also that Urtho, apparently, had the worst handwriting.