Old Monastery of San Juan de la Peña

To visit it inside you have to buy a ticket that costs 12 euros and gives access to the Old Monastery, the New Monastery and the Romanesque church of Santa Cruz de la Serós.
Tickets are available online (here: articketing.vocces.com/) but tickets that have not been sold online for that day are also sold at the box office (right at the door).
When I was there, the person at the ticket office lent himself to keep me and watch my bike while I went in to visit him.
Needless to say, it is worth it because it is impressive how the construction is integrated into the rock.
In addition to the Camino de Santiago, it is also a fundamental crossing point on the Route of the Holy Grail, as it is said that the monastery housed it between 1071 and 1399.

Compulsory visit to both monasteries.

Great monastery carved into the rock, pilgrimage site on the Way of St. James / Camino de Santiago