Researchers are now certain the remains uncovered at Llanwnda near Caernarfon date to the 11th or 12th Century - but its owners remain a mystery.
The Hen Gastell - Old Castle site - was the focus of surveys and digs over a two-year period.
The final findings of the experts have now been published by the Gwynedd Archaeological Trust.
"We were investigating the site because we didn't really know one way or another whether it was an Iron Age site," said dig director Jane Kenney.
"Now we know for sure. We had 14 radio-carbon samples dates and they all came out at about the same time."
Today, the site is an impressive earthwork monument with none of the original structure standing.
But excavations clearly showed it had been a defended enclosure, with a large ditch and substantial earthen bank outside the ditch.
Holes dug to hold posts suggest it would have held a large timber building that could have been up to 65ft (20m) long.
But who did it belong to?
"It's extremely hard to say," admitted the site director.
"It's not likely to be one of the princely courts, it was somebody a little bit lower on the social scale."
One theory is that it may have had connections with Vikings known to have settled in parts of the area, with speculation it could have been linked to Bon y Dom - a castle reputed to have been built by Olaf, King of Dublin.
"But it's all guesswork - and something the historians can help with. There's still an awful lot we don't know about the site," added Ms Kenney.
An archaeological dig in Gwynedd has discovered that what was thought to be an Iron Age fort was a medieval castle.