Beyond the technical and architectural aspects that we know regarding the Old Castle, part of its history remains a mystery. Since the foundation of the village by Segovian shepherds, during the repopulation following the capture of Madrid and Toledo, the name of Manzanares appears for the first time in 1248 in documents from the reign of Fernando III El Santo, where it is recognised as "pueblo de Manzanares" (village of Manzanares). As the he territory was in constant dispute between the peoples of Segovia and Madrid during the 12th and 13th centuries, the King Alfonso X took the area of Sexmo de Manzanares for himself in 1268 and turned the town into the head of the Real de Manzanares, an area made up of 46 places. During the 13th and 14th centuries, the territory remained in the hands of families close to the Crown until in 1337, when its new owner, Juan de la Cerda, exchanged the town of Huelva for the territory of Manzanares with Doña Leonor de Guzmán, mistress to King Alfonso XI and mother of the Trastámara Dynasty.
At that time there was a palatial building in Manzanares, which is mentioned when the king sent some carpenters to the place of Manzanares in 1346 "to prepare the palaces there which we possess". Perhaps this was the origin of the Old Castle as some researchers believe, although we are not certain that it is of the same construction.
The link between the territory of the Real de Manzanares and the Mendoza family begins on the 14th of October of 1383, when King Juan I granted half of the Lordship to Pedro González de Mendoza, his "Ayo" (Tutor), "Mayordomo Mayor" (Lord Steward) and later Captain General of his armies, for having saved his life by giving him his horse in the battle of Aljubarrota (Portugal), as well as for the services rendered to his father, King Enrique II, in the war against his brother, Pedro I.
A year later, the Lordship primogeniture was created, a title confirmed to his son, Diego Hurtado de Mendoza, Admiral of the Castilian fleet. When a lordship was granted, it was common practice at the time to build a castle as a sign of power. That explains why, unlike the defenders of Alfonso XI as the builder of the Old Castle, some authors attribute the construction of this fortress to Diego. It would also be the safe place to keep the bridge toll taxes collected from cattle, travellers and merchants who crossed the bridge on their way across the Cañada Real Segoviana (Segovian Royal Glen) in the Real de Manzanares.