For a proper Broch, see gallery https://pbase.com/crisscross/broch , but in common they have the site where they command views of sea passages whence invaders could be spied, but rather poor defensive position with adjacent higher land. Their function has never been fully unravelled, but my latest thought is that the local residents would have superiority of numbers over a typical Viking galley or two so could hold the heights and only fall back within the Broch to join non-combatants in the last resort