On top of a hill that dominates the Arlanza river valley, se find Lerma. Dating back to pre-Roman times, the village reached its maximum splendour under the rule of the Duke of Lerma, favourite of Phillip III between 1598 and 1618.

The stately appearance of the historic quarter can be seen, for example, in the majestic Ducal Palace, which at present houses the Parador de Turismo. It was erected between 1601 and 1617 by Francisco de Mora, on top of the ruins of an ancient castle. With sober and elegant lines, this building is flanked by four angular towers. Across from the palace we see the huge Ducal square, which used to be fully arcaded. The most remarkable building there is the collegiate church of San Pedro, which overlooks the Arlanza river. It dates from the beginning of the 17th century and is connected to the Ducal Palace through a projecting passage. Although the exterior appearance is quite austere, inside the temple we find a valuable praying statue of archbishop Cristóbal de Rojas y Sandoval, the duke's uncle, owed to Juan de Arfe. The 1616 organ found here is one of the oldest in Spain. Also worthy of note are the hermitage of La Piedad; the monasteries of San Blas, of Madre de Dios (Mother of God), and of La Ascensión de Nuestra Señora (the Ascension of Our Lady), all three from the 17th century; and the arch of Cárcel, gate to the ancient medieval wall, remodelled by the Duke of Lerma, and turned into a prison.