Hagia Sophia, Istambul - Turkey

Dating from the sixth century, it was originally a basilica constructed for the Eastern Roman Emperor Justinian I. A masterwork of Roman engineering, the huge 30 m diameter dome covers what was for over 1000 years the largest enclosed space in the world. The church was looted by the fourth Crusaders in 1204, and became a mosque in the 15th century when The Ottomans conquered the city. It was converted into a museum in 1935. 

St. Peter’s Basilica - Vatican City, Italy

St. Peter's is famous as a place of pilgrimage, for its liturgical functions and for its historical associations. It is associated with the papacy, with the Counter-reformation and with numerous artists, most significantly Michelangelo. As a work of architecture, it is regarded as the greatest building of its age. 

The São Paulo See Metropolitan Cathedral - Sao Paulo, Brazil

Is the cathedral of the Roman Catholic Archdiocese of the city of São Paulo, Brazil. Its construction, in Neo-Gothic style, began in 1913 and ended four decades later. It was ready for its dedication on the 400th anniversary of the foundation of the then humble villa of Sao Paulo by Chief or Cacique Tibirica' and the Jesuit priests Manuel da Nobrega and Jose' de Anchieta. Despite having a Renaissance-styled dome, the Sao Paulo Metropolitan Cathedral is considered by some to be the 4th largest neo-gothic cathedral in the world.

 St. Patrick’s Cathedral, NYC - USA

Neo-Gothic-style Roman Catholic cathedral church in the United States. It is the seat of the archbishop of the Roman Catholic Archdiocese of New York.

The cathedral is built of brick, not marble because brick is stronger. However, it was then covered with marble, quarried in New York and Massachusetts.

Notre Dame de Paris - Paris, France

The early Gothic Cathédrale de Notre Dame (Our Lady) has a 12th century design but wasn't completed until the 14th. Still it is a good example of the development of the style, though the west or main portal is a bit unusual in its rigidity. Remember that this is an active church, there may even be a mass going on. Meanwhile anybody who's interested in history should check out the crypt, entered at the opposite end of the square, in which you can observe the foundation stones for buildings on the island going back to Roman times. 

La Ermita in Cali, Colombia

A Gothic-style church located in the downtown area. It was built in 1678 and then renovated in 1930.

St. Mark’s Basilica - Venice, Italy

It is the most famous of the city's churches and one of the best known examples of Byzantine architecture. It lies on Piazza San Marco (in the San Marco sestiere or district) adjacent and connected to the Doge's Palace. Originally it was the "chapel" of the Venetian rulers, and not the city's cathedral. Since 1807 it has been the seat of the Patriarch of Venice, archbishop of the Roman Catholic Archdiocese of Venice. For its opulent design, gilded Byzantine mosaics, and its status as a symbol of Venetian wealth and power, from the 11th century on the building was known by the nickname Chiesa d'Oro (Church of gold).

St. Isaac’s Cathedral- St. Petersburg, Russia

It was built in 1818 and is a major attraction in the city. It is the third highest cupola cathedral in the world. There are night time visits, too, and the view from the observation deck (separate fee)is one of the best views of the city, for those who are willing to climb 400 steps.

Seville Cathedral - Seville, Spain

The Cathedral of Seville was once judged the third largest church in the world after Saint Peter's in Rome and Saint Paul's in London, it is now arguably the largest church in the world when compared using the measurement of volume. Seville's fifteenth century cathedral occupies the site of the former great mosque built in the late twelfth century. The central nave rises to an awesome 37 metres over a total area of 11,520 square metres. The Cathedral is the final resting place of the remains of Christopher Columbus.

Aachen Cathedral, Aachen, Germany

Frequently referred to as the "Imperial Cathedral", is a Roman Catholic church in Aachen, Germany. The church is the oldest cathedral in northern Europe and was known as the "Royal Church of St. Mary at Aachen" during the Middle Ages. For 600 years, from 936 to 1531, the Aachen chapel was the church of coronation for 30 German kings and 12 queens.

Metz Cathedral - Moselle, France

Is the seat of the Bishop of Metz. It was formed in the 14th century by joining together two churches: the nave of Saint-Etienne, built in the 13th century, was attached to the north side of an older Romanesque church. It is found in the heart of the city, on the Place d'Armes, where it provides a focal point for the "centre ville".

In the 15th century, a transept and a choir were added. This nave is the third highest in France with its 41.41 meters. Only Beauvais Cathedral and Amiens Cathedral are higher.

Duomo di Pisa - Pisa, Italy

The splendid cathedral, contains artwork by Giambologna, Della Robbia, and other major artists. Fine Romanesque style with double aisles and a cupola, a huge apse mosaic partly by Cimabue, and a fine pulpit by Giovanni Pisano in late Gothic / early Renaissance style.