Those attending the service were quickly ushered off the premises.
Their evacuation came just moments after the customary two minute silence was held.
Paramedics from the air ambulance rushed to help the man but were unable to reach him in time.
Witnesses told how visitors started screaming for help, with emergency services quick to react to the initial panic.
One witness told North Wales Live that ambulance staff who had been stationed at the Y Maes were first on the scene with police.
They said two more ambulances followed shortly after.
Around an hour later an air ambulance landed in a nearby car park.
The castle was quickly cordoned off with visitors being told to leave the area.
The man’s identity is yet to be confirmed.
Although, it is thought he is from overseas.
A Welsh Ambulance Service (WAS) spokeswoman confirmed they were called to a medical emergency at Caernarfon Castle.
No further details have been provided by Caernarfon Castle.
Neither has North Wales Police commented further on the event.
It is thought the Royal British Legion had held the service at the attraction.
Caernarfon castle has held a remembrance service of some sort year on year.
The castle was built in 736 AD and has since been both a political and historical landmark through the ages.
It sits just opposite the island of Anglesey.
North Wales Live said it has approached the Royal British Legion for comment.
It added that nobody was available at the Welsh historic monument agency Cadw to comment.