The archives are the main repository of all documents, maps, and research in the Imperium. The documents are organized and thoroughly copied for distribution of knowledge. Maps are made by the archivists and stored here. Many archivists will travel and produce these maps. Once produced these are closely guarded. While the archives may seem like a fancy library, but this isn't the important projects that they work on. Each archivist must produce one set of research findings every year for the Imperium. Funds and resources are limited, but they can still work very comfortably. They work on many secret projects ordered by the Emperor and his servants.
It's fully underground under the city of Aulus. The vast tunnels make it seem you are not even under the earth. Magic torches and light make small gardens and reading rooms around the shelves of books. It's all made of stone with incredible stone work.
The Archives was built during the establishment of when the corrupt republic was in power. While it was built way make then, it certainly hasn't changed. Over time the Archives shifts slightly when the power shifts, but they have always been kept from the public eye. While it was very small back when it was founded, now is vast and powerful.