A "hill fort" is a strategically built structure built for refuge. The structure is built to follow the contours of a hill, with giant rings of ditches going around it, so that the fort itself, when looked at from an aerial view, kind of looks like a bullseye.
There are different types of hill forts, and here are some of them:
The hilltop contour -the classic hill fort; an inland location with a hilltop defensive position surrounded by artificial ramparts or steep natural slopes.
Inland promontory -an inland defensive position on a ridge or spur with steep slopes on 2 or 3 sides, and artificial ramparts on the level approaches.
Interfluvial -a promontory above the confluence of two rivers, or in the bend of a meander.