Castellet Castle, built on the remains of fortifications dating back more than 2,500 years, was built in 977 with a basic structure consisting of a watchtower and a small wall. It was a weapon as well as a food store and it had the function of controlling roads and other routes, such as the river Foix. Built by Borrell II, from the 13th century it was ruled by the Castellet family. After a period of decadence and changes of ownership, the fortress was bought in 1928 by the archivist José de Peray March, who did a certain amount of restoration work, after which his son, Josep Anton de Peray Batlle, a priest, bequeathed it in his will in 1984 to "The Barcelona Children's Hospital". Acquired in 1999 by ACESA, it was restored to its present state, and today it houses the Abertis Foundation and UNESCO's International Centre for Mediterranean Biosphere Reserves.
Tours are held for free once a month, and they require booking in advance.