National Census Records
There are few national census records for Spain. The most common one was taken between 1750 and 1754, known as the Catastro de Ensenada. Much of this census has been microfilmed. To find the microfilm numbers you will need to do a Place Search in the Family History Library Catalog.
Local Census Records
Local municipalities (municipios) took censuses of their populations periodically. In Spain you will find that the majority of the municipios began taking population censuses in the latter half of the 19th Century and continuing onto the present time.
Municipal census records are usually housed in the Municipal archives. Some have been microfilmed by FamilySearch (see Cádiz), but most have not. The best place to find local census records is through the Directorio de Archivos. Perform a search for the municipio where your ancestor's lived.
You can review the census records for Castilla La Mancha online at this site: padrones1808.jccm.es/
16th Century Census Records
Dr. George R. Ryskamp published a wonderful article on 16th Century Spanish census records in the fall issue of the 2002 BYU Family Historian.