Barcelona is full of relatively well-preserved ancient and medieval sites, but I believe the small Benedictine monastery of Sant Pau del Camp is my favorite.
The existing buildings are from the twelfth and early thirteenth century, but the original church on the site dates back to the tenth century. It may have been founded by the second count of Barcelona, Wifred II (r. 897-911), son of Wifred the Hairy. His tombstone was discovered in the monastery during sixteenth century restorations.
The original foundation was destroyed by al-Mansur in 985, and the replacement church was not begun for some time. By 1098, a local noble family of the Bell-Lloc region had completed the restoration of the church, and it was a Benedictine monastery by 1117. It was never a big monastery– seven monks at the height. But the community added a lovely, if small, cloister by the thirteenth century.
The church itself is a romanesque treasure. The tympanum of the main door is especially nice.
Overall, Saint Pau del Camp is a fantastic example of just how much of Barcelona’s medieval heritage survives. It is striking for a large city, especially one that experienced the vicissitudes of the Guerra Civil. This is not the most impressive monastery in Spain, but it is the most impressive monastery inside a very large city in Spain.
Citations: Many of the dates cited here come curtesy of Paul Freedman, “A Privilege of Pope Alexander III for Sant Pau del Camp (Barcelona)”, Archivum Historiae Pontificiae 31 (1998), 255-63.