In AD 60 or 61 Anglesey was attacked by the Romans under Paulinus; the force crossed the Menai Straits either between Aber and Beaumaris or, as suggested in Mona Antiqua 98 (99), the narrower passage between Port Dinorwic and Llanidan. The Welsh druids were overwhelmed and defeated.
Source: J.E.Lloyd, A History of Wales, vol I, 1912, p.55.
"Maes-hir-gad" (the long army's field) and "Cae-oer-waedd" (field of cold or bitter lamentation) are supposed to have been so named from circumstances connected with the battle between the Romans and the Britons. (W W Williams, 'Mona Antiqua' in Archaeologia Cambrensis 13, Third Series 1867, p283).
Various other sources suggest that the battle was extremely violent and date it to AD 60 or 61.
B.A.Malaws, RCAHMW, 04 May 2006.