The Monclova Castle was rectangular, with rectangular towers, of which vestiges remain in the north and east angles. Although they were very remodeled with later works, the oldest preserved remains of this castle dates back to the fourteenth century, being this the Homenaje (Tribute) tower and three canvasses of the walled area. The Homenaje tower is rectangular, and has two overlapping chambers and a hooded roof parapet and battlements. This tower must have been originally solid, although today it houses the main stairs of the palace.
The entrance to the castle is through a porch with round arch on columns and family shields in the spandrels. In a frieze it can be read “Completed in 1668”. The entrance leads to a large colonnaded courtyard on three sides with semicircular arches on columns and shields of the Order of La Merced in the spandrels. The chapel looks today a tableau of Castilian trend from the XV or XVI centuries, from a palace of the Basque Country. The wood doors are an example of XVII century size.