The Harleian Chronicle for the year 822 notes:

Arcem decantorum a saxonibus destruitur (Gough-Cooper, a384.1).

Translation: The fortress of the Decanti is destroyed by the Saxons (Dumville, 10).

The fortress of the Decanti was at Deganwy and this event is repeated in later chronicles without expansion. The site is probably that occupied by the ruins of Deganwy Castle (SH 7822 7945; Scheduled Monument CN016). The Saxons in question were likely led, or sent by, Ceolwulf, the king of Mercia (Charles-Edwards, 477).

RCAHMW (Battlefields Inventory), Nov 2016

Bibliography
Charles-Edwards, T. M., Wales and the Britons 350?1064 (Oxford University Press, 2013).
Dumville, David (ed. and trans.), Annales Cambriae, A.D. 682?954: Texts A?C in Parallel (Cambridge, Department of Anglo-Saxon, Norse and Celtic, 2002).
Gough-Cooper, Henry (ed.) The Harleian Chronicle: Annales Cambriae, The A Text from British Library, Harley MS 3859, ff. 190r?193r, online edition.