In the early fifteenth century, in times of Trastamara (John II and Henry IV) are carried out major works of what is now the current castle as a whole.
The Tower of John II is located on the flat part of the site, the most difficult defense, overlooking the city of Segovia, beside the moat and entrance to the grounds. In this area there was already a midsize tower from the thirteenth century which retains a beautiful window Almohad Mudejar influence.
Taking advantage of this construction, John II had a larger height and width and for much of the fifteenth century adopted the current size dramatically, becoming one of the most impressive buildings of the palace-military architecture in Spain, thanks to its height and the game their garrets ten huge flanking the parapet. This tower was used occasionally as a prison.