More than 3,000 performers are taking part in what has been promised as the "largest ever" recreation of the Battle of Hastings in East Sussex.
Re-enactments are staged each year but this is set to be the largest
English Heritage said an audience of 20,000 people was expected to attend the event on Saturday and Sunday.
Re-enactors from Europe, Scandinavia, Russia, the US and Australia were taking part, a spokesman said.
A restaging of Harold's march from Yorkshire to Sussex also took place in the run-up to the battle.
English Heritage said performers would be living on the site in 11th Century-style encampments.
Re-enactments on the actual battlefield in Hastings take place every year, but the charity said this was the largest restaging attempted so far.
"Never before has English Heritage recreated the battle on this scale," its website said.
"William and Harold will face each other on the field of battle supported by 100 mounted troops and fearsome foot soldiers as they clash swords for the crown of England."
Steven Lowe, a participant from New Zealand, said: "The feeling of being in line, defending a hill against thousands of people coming up against you, the feeling of disorientation, the feeling of the group spirit, it's wonderful."
And Warren Cummins, a Canadian enthusiast, said: "For the last several years I've been involved in re-enactment.
"It's an honour I think to come out here and recreate something that was so significant in world history."
Arrow in the eye
Edward the Confessor's death in January 1066 left an unstable country and three contenders for the throne - Earl Harold of Wessex, who was named King, Duke William of Normandy, and King Harald Hardrada of Norway.
When Harald Hardrada landed in the north of England to claim his throne, King Harold marched north to face him and defeated him at a battle at Stamford Bridge, near York.
But then the Normans invaded and the Saxon army raced back south, arriving at Battle, in East Sussex, on the evening of Friday 13 October.
The engagement began the next day and resulted in King Harold's death, believed to be the result of being shot in the eye with an arrow.
Duke William won the battle and was crowned King William I in Westminster Abbey on 25 December that year.