The house of Medrano had a presence in La Rioja since the 14th century. In 1337 Rodrigo Alfonso de Medrano, crossbowman of Alfonso XI, bought the village of Agoncillo and the Castle of Aguas Mansas in La Rioja, and started carrying out several remodelling works adapting it to the style of the 14th century. In 1392, it was owned was Rodrigo Alfonso de Algoncillo (sic.) who bequeathed it to his nephew Diego López de Medrano (mayordomo of the king), and to his heirs, the Counts of Siruela who inhabited it until the 19th century. The coat of arms of the House Medrano with the Calatrava Cross can be found on the eastern facade of the Castle, a sign of its dominion and ownership.

Declared a National Historic Artistic Monument in 1983, this castle was built in ashlar stone in various periods (13th and 14th centuries), with a rectangular floor plan and towers at all four corners, connected by curtain walls. The coat of arms of the House of Medrano with the Cross of Calatrava can be seen on the entrance located on its eastern facade, a sign of its historical ownership and lordship.

The complete restoration of the Castle of Aguas Mansas to serve as the municipal offices of the Agoncillo City Hall has been a lengthy process, beginning effectively in 1989 when the Culture Department of the Government of La Rioja, in agreement with the City Hall, decided to intervene to save it from ruin.

The significance of its restoration is understood as perhaps the most important piece of medieval civil architecture in La Rioja. The interior combines history and modernity, and it is currently restored and used as the Town Hall.