In the following link you will find more info about the history of the Castle of Zafra, the Spanish fortress where Jon Snow was born in GOT.

Unfortunately, you can only visit the outside and it is not allowed to go up the tower. But if you still decide to travel to this point in Guadalajara you will see with your own eyes the impressive castle where the main character of GOT came to this world (well, the medieval world of the show).