Deganwy's most notable feature was Deganwy Castle (now only ruins), situated 110 m above the town, which, in the 6th century was fortified as the stronghold of Maelgwn Gwynedd, king of Gwynedd. Deganwy appears to have been the capital of Gwynedd at this time, but this was later moved to Aberffraw on Anglesey. The hill on which the castle was built was fortified many times over the centuries.
It was the site of a Norman castle built around 1082 and occupied by Robert of Rhuddlan, and later by Llywelyn the Great and Llywelyn ap Gruffudd. The castle was later demolished by Edward I when Conwy Castle was built opposite so that only ruins remain today.