These northern chiefs had two frontiers to guard or to assail: the north-eastern, extending from the glens of Antrim to the hills of Mourne, and the southern stretching from sea to sea, from Newry to Sligo. This country was very assailable by sea; to those whose castles commanded its harbours and rivers, the fleets of Bristol, Chester, Man, and Dublin could always carry supplies and reinforcements. By the interior line one road threaded the Mourne mountains, and deflected towards Armagh, while another, winding through west Breffni, led from Sligo into Donegal by the cataract of Assaroe,—the present Ballyshannon. Along these ancient lines of communication, by fords, in mountain passes, and near the landing places for ships, the struggle for the possession of that end of the Island went on, at intervals, whenever large bodies of men could be spared from garrisons and from districts already occupied. In the year 1210, we find that there was an English Castle at Cael-uisge, now Castle-Caldwell, on Lough Erne, and that it was broke down and its defenders slain by Hugh O'Neil and Donald More O'Donnell acting together. After this event we have no trace of a foreign force in the interior of Ulster for several years. Hugh O'Neil, who died in 1230, is praised by the Bards for "never having given hostages, pledges, or tributes to English or Irish," which seems a compliment well founded. During several years following that date the war was chiefly centred in Connaught, and the fighting men of the north who took part in it were acting as allies to the O'Conors. Donald More O'Donnell had married a daughter of Cathal Crovdearg, so that ties of blood, as well as neighbouring interests, united these two great families. In the year 1247, an army under Maurice Fitzgerald, then Lord Justice, crossed the Erne in two divisions, one above and the other at Ballyshannon. Melaghlin O'Donnell was defending the passage of the river when he was taken unexpectedly in the rear by those who had crossed higher up, and thus was defeated and slain. Fitzgerald then ravaged Tyrconnell, set up a rival chief O'Canavan, and rebuilt the Castle at Cael-uisge, near Beleek. Ten years afterwards Godfrey O'Donnell, the successor of Melaghlin, avenged the defeat at Ballyshannon, in the sanguinary battle of Credran, near Sligo, where engaging Fitzgerald in single combat, he gave him his death-stroke. From wounds received at Credran, Godfrey himself, after lingering twelve months in great suffering, died. But his bodily afflictions did not prevent him discharging all the duties of a great Captain; he razed a second time the English Castle on Lough Erne, and stoutly protected his own borders against the pretensions of O'Neil, being carried on his bier in the front of the battle of Lough Swilly in 1258.