A library.

These... places. He had seen nothing like them. Out of the high, blackstone forest it towered; its doorway no larger than a man, lost tiny against its massive trunk. It branched, like a tree or a web of knowledge. Corridors ran straight and narrow, only tall enough to stand in, barely wide enough to walk through. Inset into the walls, bands of glass which light shone on the words within. It did not touch the ground, but hung suspended from the stone trees surrounded it. It was a creature of straight lines, no-more-us angles. Imbeautiful asymmetry it expanded, from the center where walked those who new its purpose. A thousand tiny nodes - rooms where texts branched, a skylighting each, give in lonely light like an oasis for the solitary scholar. Within the structure - circles where, if ever were a round peg fit in a square hole, this would be it. Within each room a sunken space in the center of the floor, like a disc carved in the rock.