One of Prague’s most famous attractions, the Astronomical Clock is a work of wonder. Built in the 15th century, it features multiple moving pieces, displaying the time, the date, the position of celestial bodies, astronomical cycles, and Christian holidays. The hourly show also features a parade of the Twelve Apostles, accompanied by music and a skeleton ringing a bell of keys.

Constructed in the early 16th century, the Belém Tower served as a defense structure at the mouth of the Tagus River, protecting Lisbon, Portugal, from possible invasion. The tower underwent a significant renovation in the late 1990s, and it is now one of the most popular tourist attractions in Lisbon. The tower, together with a nearby historic monastery, was named a UNESCO World Heritage Site in 1983.

Rome’s Passetto di Borgo looks like many of the city’s ancient fortification walls from the outside, but it hides a secret passageway at the top. Built as a papal escape route in the Middle Ages, the passageway leads from the entrance of the Vatican for 875 yards to the Castel Sant’Angelo, which was used by popes as a fortress and castle. Today, it serves as a museum.

Copenhagen’s colorful port district is one of the most recognizable in the world. Once a thriving commercial area, modern Nyhavn is now a leisurely scene of waterfront restaurants, jazz music, and renovated traditional row houses painted in every shade. The neighborhood was once home to author Hans Christian Andersen, who wrote many of his famous fairy tales in the several Nyhavn homes he lived in.

Each year, more than 2.5 million people venture to the Normandy coast to see a spectacular commune called Mont Saint Michel towering atop the jagged rock of a tidal island. At high tide, the abbey appears magical, surrounded on all sides by water, and at low tide, visitors can walk straight across the sand to climb the steep, winding steps to the top. About 40 monks and nuns still reside there.

At an astounding 750,000 square feet, Prague Castle holds the Guinness World Record for largest ancient castle. Built at the behest of Prince Borivoj, the castle and its grounds — which included a palace, a monastery, and three churches — was constructed beginning in the ninth century. Over the years, the castle has undergone invasions, fires, and various restorations, but despite this, many of its historical structures remain intact.

The most famous Gothic cathedral of the Middle Ages, Paris’ Notre Dame Cathedral was predated by a pagan temple devoted to the worship of Jupiter, who was the Roman version of the Greek god Zeus. Although the temple is now gone, its foundation was preserved and used in the construction of Notre Dame, which was completed in 1345.

At the heart of St. Peter’s Square in the Vatican is an 83-foot obelisk that was brought from Egypt to Rome by Emperor Caligula in 37 CE. The obelisk is even older than that, dating back around 4,000 years, and is one of eight obelisks brought to Rome by its emperors. It was moved to its current position in the square by Pope Sixtus V in the 16th century.

What is now the Vienna Zoo was once the private menagerie of Emperor Francis Stephan of Lorraine, who established it in 1752 on the grounds of his summer residence, Schönbrunn Palace. Not only is it the oldest zoo in the world, but it is also enormously popular, attracting more than 2.2 million visitors per year.

Towering above the city of Granada, Spain, is the stunning Alhambra, which means “Red Castle” in Arabic. Named for its red-colored walls, Alhambra stands atop al-Sabika hill, alongside the Darro river. The fortress was built there strategically to overlook the city in the ninth century. Mohammed ben Al-Hamar (Mohammed I) ordered the royal residence constructed within the fortress during the 13th century.

The gardens at the Palace of Versailles inspired French-American engineer Pierre L'Enfant to propose a similar layout for America's new capital city of Washington, D.C., which was founded in 1790. The diagonal streets of the city and placement of important buildings on hills were both concepts directly derived from the gardens of Versailles.

Stonehenge is perhaps the world’s most famous prehistoric monument, but its purpose is also a mystery. However, researchers have discovered that the original Stonehenge acted as a sound chamber. When people stood inside it and spoke, the surrounding stones of the original monument boosted their volume. But for someone standing outside the circle, it would be difficult to hear. Researchers believe that the site may have been used for ritual ceremonies for a small group of elites.

The architecturally impressive Hagia Sophia that stands today was completed in 537 CE and was the central church of the Greek Orthodox faith until 1453, when Constantinople was conquered by the Ottomans and renamed the city Istanbul. Much of the interior was redecorated with Islamic art, and the Hagia Sophia served as a mosque for centuries. In 1935, it was converted into a museum, but in 2020, the Turkish government reclassified the building as a mosque.

Overlooking the Danube River as one of the tallest buildings in Budapest, the Hungarian Parliament Building is one of the most popular tourist destinations in Europe, with over 700,000 annual visitors. The building serves as both a museum and a workplace. The interior is decorated with over 88 pounds of gold and features a grand stairway, ornate ceilings, and the Holy Crown of Hungary.

Today, the Louvre is the world’s most-visited art museum, but the building it occupies dates back more than eight centuries. It was originally built in 1190 as a fortress to protect Paris. As the city grew around it, the location of the fortress was no longer strategic for defense, so in the 16th century it was reconstructed to serve as a royal palace. In 1793, it officially opened as the museum we know today.

When Walt Disney visited Germany’s Neuschwanstein Castle before the opening of Disneyland, he was so inspired by the romantic palace and its stunning backdrop that he incorporated it into the design of Sleeping Beauty's Castle. Built for King Ludwig II of Bavaria in 1869, Neuschwanstein Castle opened to the public shortly after his death in 1886.

In its prime, the Colosseum was used exclusively for fights and battles between gladiators and animals. Later, the arena was flooded and used for mock naval battles, before being stripped for materials and used for a garden, storage, and more. In the 16th century, Pope Sixtus V proposed to transform the building into a wool factory, but the cost was too high.

When the king or queen is at Buckingham Palace, the Royal Standard is flown. The flag represents the Sovereign and the United Kingdom. However, when the king or queen is away from the palace, the de facto national flag, the Union Jack, is flown in its place.

No trip to Barcelona, Spain, would be complete without admiring the one-of-a-kind modernist architecture of Catalan architect Antoni Gaudí. Highlights include Parque Güell, one of the city’s largest green spaces; a fantastical multi-story home called Casa Batlló; and the massive, yet to be completed La Sagrada Familia church. Considered his magnum opus, La Sagrada Familia has been under construction since 1882.

The Acropolis in Athens, Greece, is one of ancient history’s greatest treasures, but if you plan on visiting, make sure to pack the right shoes. In 2009, authorities banned tourists from wearing high heels at the various monuments that make up the Acropolis, as the sharp soles were contributing to wear and tear of the delicate ruins. Also banned: food and drink.

According to legend, if the resident ravens ever leave the Tower of London, both the tower and the kingdom itself will fall. The legend specifies that there must be six ravens — the seventh bird is considered a spare. The ravens have been there since at least the 17th century, when Charles II mandated their protection.

The Berlin Wall was once a symbol of division, but today the longest continuous section of the wall that is still standing has been transformed into the world’s longest open-air art gallery. Stretching 0.8 miles, the East Side Gallery showcases art from over 100 artists. The gallery opened a year after the wall fell in 1990 and soon became a protected memorial site.

Ireland’s Blarney Castle is home to the famous Blarney Stone, which is said to bestow the gift of persuasive and eloquent speech upon anyone who kisses it. The castle also houses the Witch Stone, in which the Witch of Blarney has been imprisoned since the dawn of time. Although she can escape at night, visitors can rest assured knowing that Blarney Castle closes at dusk.

For 761 days, the Frank family hid away in a secret annex of a classic Amsterdam row house. The Jewish family went into hiding during World War II in 1942, and 13-year-old Anne famously documented this period in her diaries, which became the basis for the posthumous publication of “The Diary of a Young Girl.” Today, the Anne Frank House museum is one of Amsterdam’s most popular attractions.

Constructed beginning in 1173, the Leaning Tower of Pisa is one of the most recognizable works of architecture in the world. Known as the laboratory for Galileo's experiments with gravity, the site today brings approximately 1 million tourists every year to the Tuscan city. There is also a replica of the famous tower in Pisa’s sister city of Niles, Illinois.

The Eiffel Tower was built for the ​​1889 Exposition Universelle, a world’s fair to commemorate the centennial of the French Revolution. Although it was scheduled to be torn down in 1909 at the end of its 20-year lease, its builder, Gustave Eiffel, was able to convince authorities to keep the Eiffel Tower standing due to its use as a telegraphic antenna.