ah! I've had a eureka moment! There is a story of an ancient battle in Cwm Dyli - right by pen-y-gwryd - so that makes sense. I'll try and dig out the info
.... some time later....
ok, so, just for interest, Pen-Y-Gwryd itself (not the hotel, the bit of land/lake opposite) is the site of a Roman marching camp. After the Romans there were several incursions by the Irish, and Bwlch Y Gwyddel nearby is connected with that. The battle bit may have come from fisticuffs with the Romans or the Irish in reality, but in the Arthurian legends: "John Rhys records a tale when Arthur and his men pursued their enemy into the upper reaches of Cwmllan (Camlan?), called Tregalan, on the southern slopes of Snowdon, the modern Watkin Path, where they were pushed up the bwlch, or pass, towards Cwm Dyli. When Arthur's army had reached the top of the pass, the enemy let fly a shower of arrows at them. Fatally wounded Arthur fell, and his body was buried on the mountain pass so that no enemy might march that way so long as Arthur's dust rested there. The pass is called Bwlch y Saethau, (the Pass of the Arrows) and the heap of stones called Carnedd Arthur, (Arthur's Cairn) which could still be seen on the top of the pass in 1850.
After Arthur's death on Bwlch y Saethau, his men ascended to the ridge of the Lliwed and then descended the precipitous cliff-face into a vast cave called Ogof Llanciau Eryri, (the Cave of the young Men of Snowdon) above Llyn Llydaw in Cwm Dyli. Arthur's warriors are said to lie sleeping in their armour in the cave waiting for the second coming of Arthur to restore the crown of Britain to the Kymry."