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Temple of Poseidon -

Located at Cape Sounion on the Attica peninsula, the Temple of Poseidon dates back to 440 BCE. As its name implies, it was dedicated to the god Poseidon, the protector of seafarers.

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Ancient Olympia -

Olympia, a small town in Elis on the Peloponnese peninsula, is where the ancient Olympic Games were held throughout classical antiquity.

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Delphi Oracle Sanctuary -

According to Greek mythology, the Delphi Oracle Sanctuary is where Apollo spoke. In fact, the Temple of Apollo is the central focus of the sanctuary, which dates back to the 4th century BCE and is located in central Greece.

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Epidaurus Theater -

The magnificent Ancient Theater of Epidaurus is regarded as the best-preserved ancient theater in Greece. Constructed in the 4th century BCE, it had a maximum capacity of 13,000 to 14,000 spectators. Located near modern-day Lygourio, it's still used today to stage ancient drama plays.

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The Asklepieion of Kos -

The Asklepieion of Kos, whose surviving ruins date back to the Hellenistic period, after the 4th century BCE, is the site of an ancient hospital, dedicated to the god of health, Asklepios. Kos, meanwhile, is considered the birthplace of Greek physician and philosopher Hippocrates, the "Father of Medicine."

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Mycenae and Tiryns -

The archaeological site of Mycenae and Tiryns near Mykines in Argolis, north-eastern Peloponnese, comprises the imposing ruins of the two greatest cities of the Mycenaean civilization. One of the visual highlights is the extraordinary Lion Gate, the entrance to the citadel of Mycenae built in approximately 1250 BCE.

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Knossos -

The Minoan palace is the main site of interest at Knossos, an important city in antiquity and a major center of the Minoan civilization. The palace complex was significantly—and incorrectly—reconstructed in the early 20th century. Fortunately, this error doesn't distract the eye too much from the rest of the site, which is set on the outskirts of Heraklion.

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Heraklion Archaeological Museum -

The Heraklion Archaeological Museum holds the majority of artifacts unearthed at Knossos, but, as one of the largest museums in the country, also preserves the best collection of Minoan art in the world. An outstanding exhibit is the Bronze Age Phaistos Disc (pictured).

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Medieval City of Rhodes -

The citadel of Rhodes, built by the Catholic military order known as the Knights Hospitaller, is one of the best-preserved medieval towns in Europe. It stands as one of the most beautiful urban ensembles of the Middle Ages.

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Delos -

Diminutive Delos has one of the most important mythological, historical, and archaeological sites in Greece. Supposedly the birthplace of Apollo and Artemis, the island is awash with ancient landmarks, among them the Agora of the Competaliasts and the amazing Terrace of the Lions (pictured).

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Old Fortress, Corfu -

A Venetian-era gem, the Old Fortress of Corfu is one of the most impressive fortification works in Greece. An iconic 15th-century landmark, the stronghold famously repulsed all three major Ottoman sieges: the great siege of 1537, the siege of 1571, and the second great siege of Corfu in 1716.

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Zakynthos Blue Caves -

One of the most popular and advertised attractions of the island of Zakynthos, this stunning natural wonder comprises a long chamber of successive caves best explored by sea kayak.

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Palamidi Castle -

Palamidi is an impressive fortress that overlooks the town of Nafplio in the Peloponnese region of southern Greece. Built by the Venetians and completed in 1714, the castle can only be reached on foot. That means a steep 857-step climb.

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Lindos Acropolis -

The Acropolis of Lindos on the island of Rhodes is a spectacular archaeological site that contains the ruins of the 4th-century BCE Athena Lindia temples as well as the Propylaea, a huge Hellenistic stoa, or colonnade, and the Byzantine church of Saint John.

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Meteora Monasteries -

The Meteora Monasteries are six Byzantine sanctuaries perched on top of huge pillars of rock near Trikala, in Thessaly. Of great religious significance, the monasteries house priceless artifacts, including rare and precious frescoes.

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Chania Venetian Harbor -

The Old Venetian Harbor is the iconic 14th-century centerpiece of the beautiful city of Chania. It's an Instagram favorite, and lies on the northwest coast of Crete.

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Sarakiniko Beach -

There are many hundreds of outstanding beaches in Greece, but Sarakiniko Beach on the island of Milos is exceptional in that it's composed of a scenic landscape of grayish-white volcanic rock sculpted by the wind and the waves.

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White Tower of Thessaloniki -

The busy city of Thessaloniki regales the tourist with a number of visitor attractions. Arguably the most recognized of these is the White Tower, an early Ottoman structure built over Byzantine foundations. Today, the monument also serves as a museum and is an integral component of the city's historic waterfront.

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Samaria Gorge -

The Samaria Gorge is tucked away in the White Mountains' National Park on Crete. It takes on average six hours to hike this outstanding natural landmark, one of Greece's most impressive gorges.

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Monastery of Saint John -

Appearing more like a castle than religious retreat, the Monastery of Saint John surmounts the pretty, whitewashed town of Chora on the island of Patmos. Founded in 1088, the monastery serves as a pilgrimage site and a place of Greek Orthodox learning and worship.

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Ancient Corinth -

Ancient Corinth was one of the largest and most important cities of Greece. It became a major hub of trade and culture in the 8th century BCE. Corinth was home to the legendary figure Jason of the Argonauts, while its streets were actually trodden by St. Paul (who taught the gospel of Christ here).

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Mount Olympus -

In ancient times, Mount Olympus was the home of Zeus and the Greek gods. At 9,570 feet (2,917 m), this is the highest mountain in Greece, and a favorite destination with hikers.

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Archaeological Site of Aigai -

The Archaeological Site of Aigai in modern-day Vergina comprises the ruins of the ancient royal capital of Macedon. The underground Museum of the Royal Tombs of Aigai is a veritable treasure trove of priceless artifacts.

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Akrotiri -

The well-preserved ruins of the prehistoric city of Akrotiri are certainly worth investigating. Located on Santorini, Akrotiri is the site of a Cycladic Bronze Age settlement. Excavations have revealed remarkably well-preserved streets, dwellings, and numerous day-to-day objects.

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Panagia Ekatontapiliani -

Panagia Ekatontapiliani is one of the most celebrated Greek Orthodox churches. Located on the island of Paros, the religious complex, also known as the Church of 100 Doors, dates back to the 4th century and was likely built on the orders of Saint Helen, mother of the Holy Roman Emperor Constantine.

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Temple of Aphaia -

The Temple of Aphaia on the island of Aegina was founded around 500 BCE and dedicated to the goddess Aphaia. The temple enjoys an especially idyllic location, perched above pine forests in the hills overlooking the eastern side of the island.

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Temple of Olympian Zeus -

Of course, no visit to Greece would be complete without investigating Athens. So after ticking off the best sites around the country, head back to the capital to complete your sightseeing. Dedicated to the king of the Olympian gods, the Temple of Olympian Zeus in Athens was completed in 131 CE. It was one of the largest temples in the ancient world.

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Odeon of Herodes Atticus -

The incredible Odeon of Herodes Atticus is a Roman theater fashioned entirely from stone. Completed in 161 CE, this outstanding cultural landmark is today considered one of the best open air theaters in the world.

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Archaeological Museum of Athens -

For an engaging and wholly educative overview of cultural Greece, allow a good few hours to browse the Archaeological Museum of Athens. Considered one of the greatest museums in the world, this is where to gaze in awe at artifacts like the magnificent Gold Mask of Agamemnon, a funerary mask discovered at the Bronze Age site of Mycenae.

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Parthenon Temple -

And to round off your sightseeing, it has to be the Parthenon. One of the world's most recognized ancient monuments, this mighty temple is set on the Athenian Acropolis and was dedicated to the goddess Athena.

Sources: (History) (Britannica) (Visit Greece) 

See also: The 30 most impressive ancient monuments in the world

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