Dipylon

The Dipylon was the main gate in the city wall of Classical Athens. Located in the modern suburb of Kerameikos, it led to the namesake ancient cemetery, and to the roads connecting Athens with the rest of Greece. The gate was of major ceremonial significance as the starting point of the procession of the Great Panathenaea, and accordingly it was a large, monumental structure, "the largest gate of the ancient world". Erected in 478 BC as part of Themistocles' fortification of Athens and rebuilt in the 300s BC, it remained standing and in use until the 3rd century AD.

Source: Wikipedia