This plan http://www.castlewales.com/deganwy1.html shows a reconstruction of the early medieval fort. It was the seat of “Maelgwyn Gwynedd, the foremost historical figure of the 6th century in north Wales, patron of St Cybi and St Seiriol, but reviled as a drunken tyrant by the chronicler Gildas. Excavations on the western summit in 1961-66 confirmed occupation in the 5th and 6th centuries.” http://www.castlewales.com/deganwy.html
“The area below the castle is called Maesdu (Black Meadow) and was, doubtless, the site of many bloody battles. The lower ground of the later bailey may have been the site of a settlement of serfs and bondmen; while Maelgwn’s stronghold stood atop the higher of the later castle’s twin peaks. It would have been largely of wood, although the defences included some dry stone walls. These were excavated by Leslie Alcock in the 1960s. A dozen sherds of Dark Age pottery, imported from the Mediterranean, were also discovered, showing the exceptional taste and far-reaching contacts of Gwynedd’s Royal dynasty. Deganwy appears to have been first occupied during the Roman period, but was popular in the Dark Ages because it was safe from Irish raids. The place was burnt down when struck by lightning in AD 860.” http://www.earlybritishkingdoms.com/archaeology/deganwy.html
Robert of Rhuddlan built a castle at the site in 1080, which the Welsh captured, to the point that Gerald of Wales called it a ‘noble structure’ in 1191. King John burned it to the ground early in the 13th century, Llywelyn Fawr rebuilt it in 1213, and then Dafydd destroyed in advance of the English attack in 1245, to the point the English “were forced to shiver in tents”.
“The campaign of Henry III saw the construction of walls and towers, the ruins of which survive today. The castle, with towers on each hilltop and a bailey on the saddle between, had an associated borough which received a charter in 1252. It was under construction from 1245-54 but was never completely finished. As Henry became more embroiled with his own troubles, the power of the Welsh prince Llywelyn ap Gruffydd was growing. In 1263, after a long siege, he captured this outpost of English power and systematically demolished it. When Henry’s son, Edward, advanced across this territory in 1283 he camped at the ruins of Degannwy, but recognizing the greater strategic value of a riverside site and also the political impact of a castle across the river Conwy, which up until then had been the frontier of the essential Gwynedd, he founded his new castle at Conwy. Degannwy was abandoned.
The ruins visible today belong mainly to Henry III’s castle. The defences of the bailey – earth banks and ditches on the north side, the base of two D-shaped gatehouse towers, and the curtain wall hastily built by Edward I on the south – can still be recognized. The mass of fallen masonry near the base of the gatehouse is a relic of the demolition of 1263.” http://www.castlewales.com/deganwy.html
A pic from our recent visit in 2016: