Archaeologists say the discovery of a medieval archway buried in the floor of Castle Green House is the most exciting find yet at Cardigan Castle.
Experts from Cadw, the Welsh heritage body, have been called in to examine the structure while archaeologists have sealed off the site in the main hallway.
The archway looks as though it links to the medieval cellar beneath the house. It is one of the few remains of Lord Rhys's 12th century stone castle that hosted the first eisteddfod back in 1176.
Archaeologist Nigel Page said: "There are so few elements left of the medieval castle that this really is an exciting discovery. It all adds to the story of the site."
He added that it was likely the archway was the original entrance to the north tower.
The top of the archway was unearthed as workmen removed the Georgian floorboards to install cables and pipework as part of the £11m restoration.
Mr Page added that the walls of the house followed the lines of the original medieval castle as Georgian builders took advantage of the stonework for foundations.
The original stone castle was badly damaged during the Civil War with the site being filled in to create a town bowling green in the 18th century.
The castle site is due to open to the public in spring next year.