Greece
Not much remains of this once-massive temple to Zeus, though the standing columns can still give you a sense of the temple's scope.
Peru
Founded in 1579 by a wealthy window to house the second daughters of wealthy Spanish families, Santa Catalina Monastery is a city within Arequipa. With beautiful colonial architecture, winding cobblestone streets, beautiful rooms, and ornate plazas, it's one of Arequipa's top sites.
Portugal
Founded as a convent in 1389, the Igreja do Carmo was destroyed in the 1755 Lisbon earthquake. The walls of the church still stand, as do some of the arches that once supported the roof. It's worth the visit just to wander the eerie site, open to the sky. There's also a small archaeological museum in the nave and apse containing pieces from throughout Portuguese history.
Spain
The most sacred spot in Catalunya, Montserrat towers in the mountains 1.5 hours away from Barcelona. It offers both a mountain top Benedictine monastery (Montserrat) nestled in the jagged mountainside and hikes through a unique and impressive terrain of rocky peaks.
Italy | Sicily
The Chiesa Madonna della Rocca is a Norman church dating from the 11th or 12th century A.D. Originally carved from a cave, a massive rock formation still forms much of the roof. The church is worth a visit no matter how you get there, and it's possible to drive or take a bus. But we recommend walking from Taormina (about 20-30 minutes), a challenging uphill walk that's worth it for the exercise and the incredible views.
Italy | Sicily
In the heart of Siracusa's charming Oritigia neighborhood lies the ruins of a Greek temple to the god Apollo, dating from the sixth century B.C. Some of the foundation and stairs, as well as much of one wall and a few columns, still stand.
Italy | Sicily
Syracuse's cathedral is remarkable more for its history than its decor. The cathedral was built on the site of a Greek temple whose columns are still visible as elements of the structure's wall. It's worth a visit to see that alone.
Spain
The Sagrada Familia, Gaudi's most famous architectural accomplishment, is truly awe inspiring. If you see one thing in Barcelona it should be this.
Italy | Sicily
This Norman-era church in Palermo doesn't look like much from the outside—it was founded in 1143, but a rather uninspiring baroque facade was added in 1588. However, the interior is worth a visit. The highlight is the remarkably well-preserved original mosaics, likely created by Greek artists from Constantinople.
Spain
No, this is not Barcelona's most famous cathedral - that honor goes to Sagrada Familia - but this is Barcelona's huge Gothic cathedral from the 14th century (minus the 19th century facade) which played a significant part in the history of the city.