A long time ago
The oldest evidence of human settlement in the region of Cambodia dates back about 6000 years,which is a lot older than the pyramids in Egypt.
The first use of iron was in about 500 BC. The people living there were hunter/gatherers, but later turned to farming, the excess of which they traded with other civilisations from for example Thailand.
Still a long time ago, but Jesus already died
The Khmer Empire mainly blossomed during the 12th century and was the biggest empire in Southeast Asia. The centre of the empire was Ankor,a place where multiple capitals were constructed during its reign.
The empire was shortly attacked by the Mongols lead by Kublai Khan, a descendant of Genghis khan, but the Khmer king was able to negotiate a peace by giving large amounts of gold. Multiple other smaller kingdoms would follow, sometimes being part of neighbouring countries like Vietnam or Thailand, until its colonisation by France.