Angkor Wat, the Temple City. Arguably the greatest religious building ever made, and certainly the largest, it was built by the vanished Khmer empire, during the reign of King Suryavarman II (1113 to at least 1145).
The temple grounds span 1300 meters north-south, and 1500 meters east-west. Unusually, the entrance faces west in the direction associated with Vishnu. Visitors first approach a causeway across a 200 meter wide moat. On the opposite side, an entrance pavilion  spans 230 meters north-south. Its central bays include three passages that elephants could fit through for royal processions. A second causeway decorated with naga runs for over 300 meters.
It was one of life’s great privileges to visit this incredible work at a time when there were next to no visitors. While I have seen many of the world’s great buildings -Taj Mahal, the pyramids, Bagan- enjoying Angkor in almost total peace remains one of my travelling highlights.