From the longer Wikipedia page [1] - there is more information on the Spanish Wikipedia page [2].
The General Archive of Simancas (also known by its acronym, AGS) is an official archive located in the castle of Simancas, province of Valladolid, Spain. It was founded in 1540, making this the first official archive of the Crown of Castile.
The chronological evolution of the institution has been influenced by the history of the Crown of Castile. The moments of strength or withdrawal of the Spanish monarchy were reflected in the form of documents arrivals or resource shortages. A major milestone occurred in 1588, when Philip II of Spain gave the instruction to the Government of the Archives of Simancas, a key document for understanding the management of both this file and others in the peninsula. Also the damage suffered during the Spanish War of Independence had a major impact on what is now the institution.
Currently, the AGS is a cultural institution under the Ministry of Culture of Spain dedicated to conservation, cataloging and investigation with the large funds that are hosted there. These activities have brought about the reputation of the archive as one of the cornerstones of the Iberian Peninsula in what is relates to preservation and custody of documents.
The English part of the website - which is in several languages -is [3].