In one version, the soldiers had found the one place where the waters could be forded (thus showing how much the Irish Sea has risen over the past two thousand years); or else they created a pontoon of boats to act as a bridge, as told in version two. Then they hesitated in pure intimidation.
Tacitus's Annals describes why:
On the shore stood the opposing army with its dense array of armed warriors, while between the ranks dashed women, in black attire like the Furies, with hair disheveled, waving brands.
All around, the Druids, lifting up their hands to heaven, and pouring forth dreadful imprecations, scared our soldiers by the unfamiliar sight, so that, as if their limbs were paralyzed, they stood motionless, and exposed to wounds.
Paulinus basically appealed to the male pride of his soldiers, scorning them in fearing a bunch of women. Testosterone raised along with their standard, the Romans rushed onto the island. They wrestled the brands from the hands of Druidesses, then used them to set fire to the ladies and their groves alike. Elsewhere other Druids were killed with swords over their own altars.
By the time the Menai Massacre was over, everything that represented Druidism - human, site and artifact alike - was destroyed. The old Celtic faith had lost its center of operations, but the conquest couldn't be made certain immediately. Paulinus had to quickly withdraw from the slaughter, as news reached him that Boudicca begun her uprising in the south.
But it was too important to simply tear down and walk away. Places like that could be rebuilt.
Eighteen years later, Gnaeus Julius Agricola made it a priority to secure Ynys Môn, even while much of the rest of Britain was outside the Empire's control. In 78 CE, the Roman Governor of Britain completed the conquest of Anglesey and built a fortress straddling both sides of the Menai Straits.
Segontium was one of the first Roman sites to be populated with a permanent cohort of legionnaires. It was also one of the last to be abandoned, after 410 CE, trailing after the general Roman withdrawal from Britain.
Christianity had long since flooded into the area by then, brought by missionaries traveling throughout the Roman Empire. So what happened to the Druid faith then?