Of all the places in north Wales/Gwynedd, the name for Ynys Mon was deliberately changed by the English/Norman invaders to Anglesey, but it belies the fact that Ynys Mon remains resolutely Welsh, with 7 out of 10 residents speaking Welsh.  Because of its location, the populace suffered greatly over the millenia from foreign invaders, culminating with the wars of 1277 and 1282, when it was conquered as a stepping stone to Eryri, the stronghold of the Welsh princes.  After this last war, Edward deliberately razed much that was Welsh to the ground, including Llanfaes Abbey, the gravesite of Princesses Joanna and Elinor and built Beaumaris over the top of it.  In the process, hundreds of Welsh were ‘resettled’ elsewhere and English people brought in.

“Ethnic cleansing is not a new concept. When Edward I reached Llanfaes, he forced all the Welsh people to move to a new village called Newborough. However, the worst effects were felt in the towns of Conwy, Caernarfon and Beaumaris. No Welsh people were permitted in the towns and they were mostly inhabited by the English with a few people from Ireland, Gascony and Savoy. 1,500 hectares around those towns was also cleared of Welsh people in order that the colonists had fields for crops and livestock. The villages of Aberystwyth and Lleweni were similarly cleared of Welsh people.”  http://www.princesofgwynedd.com/drivingtour.asp?pid=3

The name ‘Anglesey’ is in fact a Viking word from the 10th century, indicating that the Vikings were successful enough in their sacking of the island for a place-name to stick, and be adopted later by the English/Normans.

http://www.bbc.co.uk/wales/whatsinaname/sites/videoexplorer/pages/?jumpTo=anglesey

Anglesey has some of the best farmland in Wales, is one of the flatter areas, and is also the driest region of Wales.  Thus, settlement has existed on Anglesey as long as people have lived in Wales.  Prehistoric megaliths scatter the island:  http://www.megalithia.com/overview/anglesey.html

Knowing a good thing when they saw it, the Romans conquered Anglesey in 61 AD but only after defeating Boudica elsewhere:  “The Romans vehemently opposed the Celtic druids, whom they did not see as pious priests, but as ferocious freedom fighters – terrorists. The druids continuously tried to rally the local population to take up the arms against the Romans. The Roman invasion of Britain had set these men on the run, with the centre of the druid cult becoming, or possibly always being Anglesey, which thus, in the first century AD, was the centre of the Celtic religion in Britain.

This situation is confirmed by the Roman historian Tacitus and Emperor Nero, who specifically identified Anglesey as an island that needed to be conquered. Many troops were relocated from other British locations towards Wales in an effort to do so. However, this power vacuum elsewhere resulted in certain insurrections, such as that of Queen Boudica.

Realising the Roman troops could not maintain order and attack Anglesey at the same time, the Empire forsook a final attack on Anglesey – the conquest of Anglesey was insignificant against the loss of London and the rest of Britain. Hence, it is claimed that the Roman general Paulinus tore up Nero’s orders, returned to London via the newly constructed Watling Street, to meet the army that had been scrambled by Queen Boudica, which had left London, in search of a Roman army they could fight. In the end, the battle occurred in Atherstone, Warwickshire, where the Romans attained an easy victory. Enthusiasm lost against well-oiled organisation.

The fact that “druid terrorists” lived in Anglesey meant that in 61 AD, Suetonius Paulinus managed to get his army across the Menai Strait and massacred the druids and burnt their sacred groves. The Romans remained aware, however, that the druids might continue to pose a problem and hence they constructed the fortress of Segontium, present Caernarfon, on the edge of the Menai Strait, to make sure that what little remained of an intact Celtic culture remained on Anglesey – and did not try to seed dissent in “Roman Britain”.

Tacitus wrote how the battle occurred on the coastline of the Menai Strait: “On the coastline, a line of warriors of the opposition was stationed, mainly made up of armed men, amongst them women, with their hair blowing in the wind, while they were carrying torches. Druids were amongst them, shouting terrifying spells, their hands raised towards the heavens, which scared our soldiers so much that their limbs became paralysed. As a result, they remained stationary and were injured. At the end of the battle, the Romans were victorious, and the holy oaks of the druids were destroyed.””

http://www.philipcoppens.com/anglesey.html

After the Romans, came the Irish, the Vikings, the Scots, and the Danes (briefly), but it was strong enough defensibly for the Kings of Gwynedd to seat their court on the west coast at Aberffraw from c.860 AD until c.1170 AD.  No trace remains of that court as the llys was dismantled for the building and maintenance of Edward I’s castle at Beaumaris.  http://en.wikipedia.org/wiki/Aberffraw