Archivo Histórico Nacional

From the Wikipedia page

National Historical Archive (Spain)

The National Historical Archive of Spain is based in Serrano Street in Madrid. It was founded in the nineteenth century when it shared a building with the Real Academía de la Historia.

The collections of the Archive include sections housed outside Madrid, for example the General Archive of the Spanish Civil War in Salamanca.

See also List of archives in Spain

The website, in Spanish, is