There has been a longstanding association, in antiquarian writings, between Cadwallon and a possible Llys (medieval royal court building) known as Caswallon's Llys. This was indicated on the Ordnance Survey map of 1889 as within a field near Mynnydd Eilian, in the Llaneilian community, in the north-east corner of the Isle of Anglesey. With no obvious remains by the 20th century, it had been largely discredited as a Llys site until a geophysical survey in 2009 identified foundations of a rectangular building within a trapezoidal enclosure, for which an early medieval site was a strong possibility.
