Tier-Group One, Tier-Group Two, Tier-Group Three

[Tier-Group 3, Control Room]

"This Library-City Dungeon is built by stacking up several thousand libraries, and from the top down they run, in order, Tier-Group 3, 2, 1, 36, 37, and so on. In other words, Tier-Group 1, which was here from the very beginning, is third from the top. The original from which this Tier-Group 1 was replicated is a library out in the countryside of Saitama. It was originally the city's library training center, so it's fitting, I suppose, as the 'first' library."

(This is the library that was here from the start.)

"According to the data, in terms of scale it's a single-story building of about 500 square meters with a capacity of 80,000 volumes. In the otherworld it opened in 1992."

"That's quite old."

"Since it's replicated and summoned as a dungeon structure, there's no problem on the point of structural strength."

"Tier-Group 2 is the city library. A reasonable choice, I think."

(At this stage there was only one library to choose from; there was no real option.)

"Then Shiso (Asura) were added to the staff. The first job they were made to do was electronically linking Tier-Groups 1 and 2, which had different systems."

(The cause was that they'd been mistakenly summoned from a different otherworld.)

"In scale it's four stories above ground and one below, about 3,500 square meters, with a capacity of 300,000 volumes. It opened in 1973. The fourth floor is a penthouse, so effectively it's a three-story building."

(At first we used it for residential quarters, but it was too far from the ground and took too long to reach, so it now stands empty. Being too high is a problem in its own way.)

"And then, as for Tier-Group 3, if one were to choose reasonably it ought to have been the prefectural library. However, the detailed data on the prefectural library in the Dungeon Core was from the old four-library-system era, and there was no information on the newer libraries. Presumably the data was lost when the Director-dono 'botched things.'"

"Is it not in the books we summoned?"

"It is in the books, of course, but since the information can't be searched within the Dungeon Core, it can't be replicated and summoned."

(It may simply be that the Core's information was outdated from the very start.)

"And so it was a university library, wasn't it."

"As with ordinary libraries, university libraries of privately-run institutions are in principle outside the scope of summoning, so the candidates for summoning are restricted."

Just as with private hotels and shopping malls, if there's a branch reading room of the city library inside it, a private school's building can be summoned.

"Furthermore, a city library casts a wide net, since all citizens are its users, whereas a university library is basically for its students and faculty, so its summoning cost is generally on the higher side. As for prefectural libraries, perhaps because their users expand to include all the prefecture's residents, the summoning cost doesn't increase by much. The summoning cost of a public library is, roughly, proportional to the floor area of the entire building containing the library."

(Strictly speaking it's probably some convoluted formula, but as long as dungeon energy can't be expressed numerically, we can't know the details.)

"University libraries, then. The universities found in most worlds are easy to summon, you said."

"The basic summoning candidates that exist in most worlds are 23 schools: 7 Imperial universities, 12 government-established universities, 2 national women's universities, and 2 public universities. In the otherworld with which this dungeon has deep ties, one more Imperial university still survives, and Osaka Imperial University and Kobe University of Commerce have been merged, though."

Wholly new-system universities raise the summoning cost because "in many worlds they don't even exist."

(Also, there were various conditions: that countryside far from Tokyo was cheaper, that Imperial universities were absurdly expensive while public ones, conversely, were cheap, and so on, so at the time there was no choice. Considering the side effect that the Shiso (Asura)'s setting-based academic background would be locked in after the fact, perhaps, if not for the matter of the plague, we ought to have endured a little longer and saved up dungeon energy.)

"And so, in the early days, this is roughly how things were. After that, we defeated bandits and enemy armies and expanded the dungeon. And there you have it."