In 61 AD a Roman army is traditionally believed to have been ambushed at Nant Sepyr (the pass beneath Bryn Euryn), and its second in command (Sempronius) killed in the fighting.
Whist the Romans were attacking Anglesey and suppressing the druids based there, a major revolt by the Iceni broke out in the South led by Boudicca. They defeated the Roman ninth legion, destoyed the then capital of Roman Britain (Colchester) and went on to destroy London and St Albans, massacring their inhabitants. There was a real danger that the Romans would be driven out of Britain and their invasion defeated. On receiving the news, the Roman Governor in command of the legions fighting in North Wales hurriedly brought them back by forced marches to confront and ultimately defeat Boudicca. To quote Tacitus, a contemporary Roman historian, “If Suetonius had not hastened back by rapid marches, Britain had been lost.” In another passage he writes “A more active campaign has never been known, nor at any time was Britain so fiercely disputed. Our veteran forces were put to the sword, our colonies smoked on the ground; and the legions were intercepted on their march”. This accords with the traditon that Sempronius (their second in command) was killed in an ambush at Nant Sempyr (hence the name) during this forced march. It is the natural place for an ambush, as it is the natural route to Chester from Anglesey (the modern A55 and the railway both travel through this narrow pass today), and it would have also have been densely wooded then to provide good cover.
