Anglesey or Ynys Môn is an island off the north west coast of Wales. Two bridges span the Menai Strait, connecting it to the mainland: the Menai Suspension Bridge designed by Thomas Telford in 1826 and the Britannia Bridge. Formerly part of Gwynedd, Anglesey, Holy Island and other smaller islands now make up the Isle of Anglesey County.
Almost three quarters of the inhabitants are Welsh speakers and Ynys Môn, the Welsh name for the island, is used for the UK Parliament and National Assembly constituencies. With an area of , Anglesey is the largest Welsh island, the fifth largest surrounding Great Britain (the largest outside of Scotland) and the largest in the Irish Sea.
Numerous megalithic monuments and menhirs are present on Anglesey, testifying to the presence of humans in prehistory. Plas Newydd is near one of 28 cromlechs that remain on uplands overlooking the sea. The Welsh Triads claim that Anglesey was once part of the mainland.
Historically, Anglesey has long been associated with druids. In AD 60 the Roman general Gaius Suetonius Paulinus, determined to break the power of the Celtic druids, attacked the island utilizing his amphibious Batavian contingent as a surprise vanguard assault and then destroying the shrine and the sacred groves. News of Boudica's revolt reached him just after his victory, causing him to withdraw his army before consolidating his conquest. The island was finally brought into the Roman Empire by Gnaeus Julius Agricola, the Roman Governor of Britain, in AD 78. During the Roman occupation, the area was notable for the mining of copper. The foundations of Caer Gybi, a fort at Holyhead, are Roman, and the present road from Holyhead to Llanfairpwllgwyngyll may originally have been a Roman road.
British Iron Age and Roman sites have been excavated, and coins and ornaments discovered, especially by the 19th century antiquarian, William Owen Stanley. Following the Roman departure from Britain in the early 5th century, pirates from Ireland colonised Anglesey and the nearby Llŷn Peninsula. In response to this, Cunedda ap Edern, a Gododdin warlord from Scotland, came to the area and began the process of driving the Irish out. This process was continued by his son Einion Yrth ap Cunedda and grandson Cadwallon Lawhir ap Einion, the last Irish invaders finally being defeated in battle in 470. As an island, Anglesey was in a good defensive position and, because of this, Aberffraw became the site of the court, or Llys, of the Kingdom of Gwynedd. Apart from a devastating Danish raid in 853 it was to remain the capital until the 13th century, when improvements to the English navy made the location indefensible.
After the Irish, the island was invaded by Vikings, some of these raids being noted in famous sagas (see Menai Strait History), as well as Saxons, and Normans, before falling to Edward I of England in the 13th century.