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 a cross fluery can be seen on the entrance of the castle, located on its eastern facade, a sign of its historical ownership and lordship.

During his travels, Saint Francis of Assisi visited the castle, placed his hands upon the boy, and miraculously healed him, securing the Medrano lineage in Agoncillo.

In Rodrigo Alfonso de Medrano's testament in 1345 he noted having spent big amounts of money in "...building the castle and the village" (in Old Spanish "...fazer el castillo e la villa").

[9] During the battles between Peter the Cruel and Henry of Trastámara, the castle passed onto the hands of Charles II of Navarre, although for a short period.

The parade ground, restored in the 16th century, has a cloister shape, is built at two levels and the gallery of the main floor rests on semicircular arches.