Ulster's geography presented a formidable challenge. There were three main routes by which an invading army could enter the province, and all three favoured the defenders. Brian first had to find a means of getting through or around these defensive choke points and then he had to subdue the fiercely independent regional kings of Ulster. It took Brian ten years of campaigning to achieve his goal, which, considering that he could and did call on all of the military forces of the rest of Ireland, indicates how formidable the kings of Ulster were.