Archaeologists believe they have unearthed part of a medieval drawbridge at Carmarthen Castle.
The work has been going on at the castle for two years
The discovery means experts now think parts of the west Wales town were well below the ground level of some of the streets of today.
The landmark has been the focus of a £1m dig and restoration project that is drawing to a close after two years.
But the discovery has boosted hopes more money will be forthcoming from the Heritage Lottery Fund for further work.
Piers thought to have supported the drawbridge have been found near Bridge Street.
This follows on from the discovery of a network of medieval walls, cellars and cobbled floors earlier in the year.
Pottery, leather shoes and other items have also been uncovered at the site.
The piers, which extend 25 feet below street level, would have been built as part of a causeway across the castle moat.
Archaeologists are dating everything they have discovered before preparing a detailed report on the excavation works.
The area in front of the castle will now be filled in and covered with a temporary surface while the county awaits the result of a lottery bid for extra funding for the final phase of works.
Building conservation officer John Llewelyn said: "The archaeologists have finished their work and are dating everything they have unearthed.
"They have found structures in front of the castle which show there was originally a drawbridge there.
"We are now in talks with the Heritage Lottery Fund over the application for funding for the fourth and final stage of works.
"It is a detailed process of negotiations and it will be some months before the public see any new work starting."
The castle, which dates back to the twelfth century, was officially re-opened in the summer after major enhancement works.
About three-quarters of the cost was met by a lottery heritage grant, with the remainder coming from Carmarthenshire Council and the Welsh ancient monuments body Cadw.
The castle was founded in 1109 by Henry I and became the centre of Norman control in south west Wales.
Much of the building was destroyed by the Owian Glyndwr rising in the fifteenth century, before being strengthened during the civil war of the 1640s.