Palacio de los Guzmanes (Palace of the Guzmans), Ducal Palace of Medina Sidonia, has belonged to the family since 1297. Granted together with the town of Sanlúcar de Barrameda to Alonso Pérez de Guzmán, Guzmán el Bueno, by King Ferdinand IV in accordance with the promise made to his father King Sancho IV for the defence of Tarifa. It was designated a Historic-Artistic Monument in 1978.

The palace has valuable collections of civil and religious art, among the most important in Spain, with paintings and sculptures by European artists; an interesting collection of Baroque furniture; large gardens and, since 1962, the General Archive of the Medina Sidonia family (AGFCMS), thanks to the initiative and hard cataloguing work of Luisa Isabel Álvarez de Toledo y Maura, the 21st Duchess of Medina Sidonia, it is now a research centre comprising over six million documents in 6,317 files belonging to different noble families or stately homes that joined the house of Medina Sidonia. It is now one of the most important privately-owned historic archives in Europe.

Furthermore, the Casa Medina Sidonia Foundation runs a 9-bedroom guesthouse known as the hospedería, located on the palace grounds, inside a 15th century building, as well as a café in the area of the Arab ribat or frontier post, dating to between the 11th and 12th centuries.

The Casa Medina Sidonia Foundation seeks to conserve, disseminate and promote its historical and artistic heritage by acting as an instrument of research and knowledge.