The Violet City, like many of the older cities of the lands, is built up over the ruins of the structures that came before it. While many buildings boast basements, they are often the basements of prior structures, or even the prior structures themselves.
The Great Library's massive stone construction and great age of course brings to min rumours of it having links and secret doors into the ancient underground ruins of the city and from there in turn to the deeper recesses where foul things summoned during the great war still hunt the prey they were summoned to kill.
For those who have gained access to the basement of the structure via the secret trap door in the civic law and economics wing are usually disappointed to discover that the basement follows the same general layout as the library itself and is evidently of the same age as the rest of the structure.
The main chamber of the basement is a mix of old books and stored antiquities of dubious value but of enough interest to keep around. This includes two different collections of texts in languages still unknown (use of read magic spells allowed for their translation years ago, but the content is still so esoteric and confusing that the originals have been kept just in case), a small library of maps of locations that no one has managed to identify, and a shelf of codices detailing magics used by no known spellcasters. And of course some well-hidden alcohol for those events when less senior librarians wish to celebrate a discovery.
Three locked chambers are set to the west of the main chamber, each supposedly to a reserved stack of documents. The northern chamber contains the cream of the crop of works of art and science from the elven empire prior to the great war. Each book, tablet and scroll looted from elven fortresses and libraries with many still bearing the marks of the war - bloodstained, damaged and partially burned.
The southern chamber in turn contains what parts of the history and culture of the clans of dwarves who arrived during the war from their own worlds via the mighty earth ships. The chamber these texts are kept in is very roughly cut from the stone and seems out of place as the home to dwarven texts. Unknown to all but a few of the eldest librarians, the mind of a ruined dwarven earth ship has been embedded into the stone pillar in the centre of the chamber. Without earth ship nor clan brothers the mind is definitely unhinged if not fully insane, but it still offers nuggets of truth and key anecdotes on occasion.
The central chamber on the west wall contains a number of works of sculpture from various other worlds. Statues and statuettes of strange creatures, plants and godlings form the heart of this collection and all watch over the stairs that lead down to the secret sub-basement.
The sub-basement of the library is exactly what you would expect to find in a normal library's basement - books that no one cares about or understands slowly rotting away and covered in dust, mixed in with the occasional incredible gem that no one realized has the answer to everyone's important questions and how to save the world, or something close to that.
One full shelf down here is dedicated to a previous chief librarian's obsession over ancient prophecies that failed. Doomsday texts dated for hundreds and even thousands of years ago, descriptions of young farmboys who would save the world (or damn it), and apocalyptic visions of the giant worms eating the world from the inside out during the hundredth annual celebration of the founding of the Violet City.
One door in the sub basement has been sealed - boarded over, barred and wizard-locked. It leads to another room like the rest down here, but one with a breach into the ruins of the old undercity. This room was originally breached from the other side by a group of thieves and adventurers seeking materials of the elven sorcerer kings from the basement stacks that they could use to open a portal to another world where the elves still live in their misty dells and practice potent magical artifice... Evidently the undercity here can be accessed from other points in the area - probably not from a structure on Guilder's Way, but from other structures nearby and also probably from either of the city's thieves' guilds.
You can, as always, download the maps of the Great Library's basement and sub-basement at 300dpi (both with and without grid) from the blog post at http://wp.me/psUVp-2h6