The most significant difficulty for English forces in confronting O'Neill lay in the natural defences that Ulster enjoyed. By land there were only two viable points of entry to the province for troops marching from the south: at Newry in the east, and Sligo in the west – the terrain in between was largely mountains, woodland, bog, and marshes. Sligo Castle was held by the O'Connor sept, but suffered constant threat from the O'Donnells; the route from Newry into the heart of Ulster ran through several easily defended passes and could only be maintained in wartime with a punishing sacrifice by the Crown of men and money.