It can't see her.

She steps into the dungeon to find out, from a blank slate of zero backlash, how hard it is trying to see her.

Nope.

...Or, well, the birds and the bugs have a bird-and-bug normal amount of trying to perceive her? But they aren't trying particularly hard to perceive her the way a dungeon monster might. None of them are even trying as hard as a mosquito would be to find a snack.

Huh. It's not trying to see her, it's not trying to mind-control her, she could just be hanging out in a forest arbitrarily insisting on being invisible for no reason. That at least means she doesn't need to be in a huge hurry to case the joint.

How big is the dungeon?

It's a little over a hundred acres, in a very tidy circle. 

It isn't unbroken forest--there are streams that start at the edge and flow inwards, or flow to the edge and disappear, and a few of those have ponds within seeing distance even with the trees reducing that quite a bit. The ground isn't perfectly level, either; there are hilly bits of varying sizes, and rocky outcroppings, and so on. Also, she may notice that it doesn't stay the same kind of forest throughout; there are clumps of different kinds of trees, and a correlation between different trees and different birdsongs. 

She takes lots of pictures and some video, pacing the whole circumference, and then makes sure the exit still works, and then starts spiraling in toward the center.

The exit still works! 

About a third of the way in, the treeline starts thinning out. 

May is not actually excited to murder a dungeon that hasn't done anybody any harm, but it won't hurt to know where it is, just in case. She is being racist against dungeons but she has a really good reason, okay. Inward and inward.

As soon as the trees have become sparse enough to give her a clear view inwards, she sees the castle. 

It's a very traditional--perhaps even stereotypical--Western construction; pale grey stone forming towers that quite clearly resemble chess rooks, with stone walls stretching between them, and a taller tower rising from the center, a red and white pennant flapping in a wind she can't feel at ground level. 

Pretty. It will be sad if they have to kill it. They probably will. It's in an unusually low population density area for a dungeon but you don't want to evacuate even a small town if you don't have to, and nobody's going to want to live near a dungeon even if it looks like it's just hanging out. Is the pennant in a recognizable pattern at all? Has the castle got a door?

The pennant is a white field with a red object taller than it is wide on it, but it's hard to see more details than that while it's whipping around and at a distance. The castle has a big portcullis on one side--the front, presumably--and also smaller, less obvious doors in assorted other locations. 

There's a dirt road extending from the portcullis, that goes for a little ways, splits and merges to form a circle around an alabaster platform, and meanders off a ways before terminating not far from the end of the treeline.

Anything sus about the alabaster platform besides that it exists in a dungeon? Is the portcullis open?

The portcullis is open. 

The platform is tall enough that you can't clearly see what's on it from the ground, but if she climbs its terraced rings, she'll find--a girl. 

The woman is perhaps a little older than May, but not by a whole lot, and she's definitely not a kidnap victim. The wings that stretch out beside her put paid to that idea, as do her pointed, elf-like ears. 

She's fast asleep, her hands folded over her chest, a wand in the same white with red accents palette as the pennant and her dress held beneath them, and a small white dragon-like creature snoozes curled up on her belly.

May is almost completely not backlashed, because being invisible to bugs is not very expensive, but she can sure imagine being backlashed enough to try to wake this entity up and say hello.

She takes a picture, and another one focused on the dragon.

Is there anything else in the castle?

The castle has a number of rooms, many of which contain normal castle accoutrements like beds and chairs. Some of them also contains non-normal castle accoutrements, like hot and cold running water, in the case of what are clearly bathrooms. The castle has a cellar, full of various comestibles; it does not have a visible dungeon.

The central tower--the keep--contains a reasonable supply of things like bedrooms, bathrooms, and kitchens, but most of its space is dedicated to books. Lots of books. Lots and lots of books. 

...Relatively few of them in languages May can read, though. A small number are in English, and smaller numbers in other languages she recognizes as being either real or based on real languages by someone who had any idea what, say, Arabic, looks like. If she searches she can find a few in French. But a solid majority are in what look to be completely made-up scripts.

The keep has a basement, but the door to it is locked.

The courtyard, between the outer wall and the keep, is filled with marble pedestals, each with a ?glass? object in a unique shape and color on it. 

She might decide to walk through the door to the basement but that's best saved for the end of this excursion.

She's never heard of a dungeon with books in it, and looking at the titles will cost her fuckall since there aren't even any animals inside the library; what are the titles, of the English and French ones?

Does anything happen if she tries to collect objects? A walnut, a book, a glass thing? The glass things are especially good candidates for being industrially useful dungeon materials.

The French ones are a book of poetry and what seems to be some sort of expedition log; English ones run the gamut between real literature written prior to the twentieth century to diaries to what claim to be grimoires of magic. 

If she tries to pick up a glass thing it is SUDDENLY NO LONGER THE CASE THAT THE DUNGEON IS NOT PAYING ATTENTION TO HER. 

WHAT IF SHE PUTS IT RIGHT BACK

Oh it immediately stops caring. 

Okay. That's another thing best saved for end of run, then. This... might be coming up on the end of the run, actually? She'll go phase through the basement door to see if there's cool shit down there.

There are corpses down there. 

...Not of kidnapping victims. The bodies in the basement, each one laid out on an alabaster slab with a thin sheet of fabric draped over them, all have the same wings-and-pointy-ears thing going on that the girl outside did, but they aren't breathing, and the basement is chilly enough to see her own breath.

No dungeon core, though.

It would actually be a little less weird for a dungeon to put wings and pointy ears on people than for it to have readable books in normal languages. She pulls the fabric back to get pictures of all their faces.

There are two women and two men. One of the women is as pale as the girl outside; the other woman and both men have dark hair.

All four of them have some level of plausible family resemblance to the girl outside, although obviously it's strongest in the white-haired woman.

Well, that makes it less likely they'll be identifiable, if their faces are also getting changed to match the sleepy boss monster. She politely replaces all the fabric. Is there another level of the dungeon, "four" would be a surprising number.

Unless she has a particular advantage at finding hidden doors she cannot be certain it does not. But it's not identifying any such to her. 

She has no particular secret door expertise. Back up - can she let herself out from this side or does she have to go insubstantial again?