As the Roman power failed in Britain, Mona became the stronghold of the invading Gwyddyl or Irish; they held it, and erected on its commanding heights their stone-walled fortresses, and it was not till the time of Caswallon Long-hand, grandson of Cunedda, that they were dislodged. He fought them in a series of battles, drove them from their strong castles faced with immense slabs of granite, such as Tin Sylwy, swept them together into Holy Island, then broke in on their last remaining fortress. According to legend, Caswallon was obliged to fasten his Britons together with horse-hobbles, to constrain them to fight by taking away from them the chance of escape by running away. With his own hand he slew Serigi, the Irish chief, near the entrance to the camp, and those of the Gwyddyl who did not escape in their boats were put to the sword. By an odd freak much like ours in glorifying De Wet and Lucas Meyer, the Welsh agreed to consider their late enemy as a martyr, and a chapel was erected where he fell, and he is figured, very shock-headed and bearing the short sword wherewith he was killed, in a niche of the doorway of the church which now stands in the midst of the old Gwyddyl fortress. Caswallon set up his residence on the hill above Llaneilian, where the foundations may still be traced--a spot whence in the declining day the mountains of Wicklow may be seen, the Isle of Man stands out to the north, and in clear weather Helvellyn may be distinguished on the rim of the blue sea. Edwin, king of Northumbria, conquered both Mona and the Isle of Man in 625. The place of his landing is still pointed out at Lleiniog, near Beaumaris, and a mound of the Anglo-Saxon type remains to show where was his first camp. Here also Hugh the Fat, Earl of Chester, was killed by the arrow of Magnus Barefoot. But of this more presently. Driven from Deganwy, on the Conway, the kings of Gwynedd made their residence at Aberffraw, in Mona. Of that palace there are but scanty traces. There is something remarkable in the character of Anglesey. The bold mountains of Wales come to an abrupt fall at the Menai Straits, and thence the island stretches west in low undulation rising nowhere to any considerable elevation, and scored across with depressions from north to south, feeble and imperfect replicas of the Menai Straits. One is the furrow occupied by the Malldraeth morass and sands, but this does not cut completely across the island. The other is more thorough; it severs Holy Island from the main body of Môn, but it is so narrow that it has been bridged at Penybont and the railway crosses it on a causeway at Valley.