The temple the exiles rebuilt after the return from Babylon was modest, and by Herod's time it was nearly five centuries old. Around 20 BCE he began not a repair but a reconstruction on a scale nothing in the region matched. The Temple Mount platform was roughly doubled by building enormous retaining walls out from the natural hill and filling behind them, producing an artificial esplanade of about fourteen hectares, the largest sacred precinct of the ancient world. Josephus reports that the sanctuary itself was rebuilt by priests trained as masons so that no unconsecrated person entered the holy area, and finished in about eighteen months, while the surrounding courts and porticoes took eight years and work on the complex continued for decades after his death.
Herod's political problem shapes the whole project. He was an Idumaean, from a people forcibly converted to Judaism only a generation or two earlier, installed by Rome after a siege of Jerusalem, and married into the Hasmonean house whose surviving members he then killed. He could not be a legitimate king in the eyes of many of his subjects, and the temple was the most conspicuous answer available: whatever else was said of him, he had built the house of God more magnificently than Solomon.
Everything above the platform was destroyed by Roman forces in 70 CE. The retaining walls remain, and their western face is the Western Wall, the holiest accessible site of Judaism, so that the most venerated surviving structure in Jewish practice is the substructure of a building project by a king whom the tradition largely despised. The lower courses of enormous drafted-margin ashlars, some weighing many tons, are Herodian work.