10 most difficult ar­chi­tec­ture riddles

Washington Union Station is a major train station, transportation hub, and leisure destination in Washington, D.C. Opened in 1907, it is Amtrak's headquarters and the railroad's second-busiest station with annual ridership of just under 5 million and the ninth-busiest in overall passengers served in the United States. Daniel Burnham, a famed Chicago architect wrote; "The station and its surroundings should be treated in a monumental manner, as they will become the vestibule of the city of Washington, and as they will be in close proximity to the Capitol itself."

Whit 162,000 sq meters of floor area, the Palacio Real is more than twice as large as Versailles, Buckingham Palace or Royal Palace in Stockholm. It is the largest functioning palace in Europe (however, king of Spain doesn't live here any more).

Mysore is commonly described as the "City of Palaces", and there are seven palaces including this one. The current structure was constructed between 1897 and 1912, after the Old Palace was burnt ablaze. Ambavilas Palace is one of the most famous tourist attractions in India, after the Taj Mahal, with more than 6 million annual visitors.

The Palace of the Grand Master of the Knights of Rhodes also known as the Kastello, is a medieval castle in the city of Rhodes, on the island of Rhodes in Greece. It is one of the few examples of Gothic architecture in Greece. The site was previously a citadel of the Knights Hospitaller that functioned as a palace, headquarters, and a fortress.

Le Corbusier was one of the most influential and controversial artists of the 20th century, and the pioneer of what is now called modern architecture. His five-decade career reshaped cities from South America to India. In 2016, seventeen of Le Corbusier's buildings, spanning over seven countries, were inscribed to the UNESCO World Heritage Sites list.

The Selimiye Mosque is an Ottoman imperial mosque, which is located in the city of Edirne, Turkey. The mosque was commissioned by Sultan Selim II, and was built by architect Mimar Sinan between 1569 and 1575. It was considered by Sinan to be his masterpiece and is one of the highest achievements of Islamic architecture.

Castel del Monte is a 13th-century citadel and castle situated on a hill in Andria in the Apulia region of southeast Italy. It was built during the 1240s by the Emperor Frederick II, who had inherited the lands from his mother Constance of Sicily.

Las Lajas Sanctuary is a basilica church located in the southern Colombian, and built inside the canyon of the Guáitara River. The present church was built in Gothic Revival style between 1916 and 1949. The name Laja comes from the name of a type of flat sedimentary rock similar to shale and slate.It rises 100 metres high from the bottom of the canyon and is connected to the opposite side of the canyon by a 50 metres tall bridge.

The Corinthian order is the last developed of the three principal classical orders of ancient Greek and Roman architecture. It is the most ornate of the orders. Its style is characterized by slender fluted columns and elaborate capitals decorated with acanthus leaves and scrolls. The oldest known example of a Corinthian column is in the Temple of Apollo Epicurius at Bassae in Arcadia, c. 450–420 BC.

The temple - 43 meter in diameter - was built by emperor Hadrian. Its dome with 8-meter wide oculus was built of unreinforced concrete that has been a mystery for ages. The weight of Roman concrete used to build the monolitic structure is merely 1/5 of common concrete. Such effect was achieved by mixing in of volcanic tuff and pumice.