* Castell Dinas Bran where the survey is taking place.
Denbighshire County Council is carrying out a geophysical survey at Castell Dinas Bran in Llangollen to see if there are any structures underground.
The work is being done thanks to funding from Cadw and The Castle Studies Trust.
A geophysical survey involves the use of probes and magnetism techniques which will allow the council to get a picture of what is within the castle and hillfort without disturbing the ground at the protected monument and site of special scientific interest.
Work started this week and is being undertaken by specialist company Tigergeo and the results should be available within a few weeks.
County archaeologist Fiona Gale said: “This is a great opportunity for us to try and find out more about this fantastic site which has been an important place in the Clwydian Range and Dee Valley Area of Outstanding Natural Beauty for thousands of years.
“It will be intriguing to know whether any remains of the 2,500 year hillfort are evident or did the castle obliterate it all? We think there must have been more buildings within the castle walls but we’ll have to wait and see whether there are any hints of their remains.
“Hopefully, the work will allow us to know a little more about the castle and that will help us care for it in the future and interpret it to enhance the visitor experience.”
Jeremy Cunnington, chair of trustees at the Castle Studies Trust, said: “We’re delighted to co-fund this survey work to advance our understanding of Dinas Bran which is an iconic and vitally important native Welsh Castle."
* To learn more about these and previous projects the trust has funded people can visit the Trust’s website: www.castlestudiestrust.org