Univallate hill fort

A hillfort or hill fort is a type of earthworks used as a fortified refuge or defended settlement, located to exploit a rise in elevation for defensive advantage. They are typically European and of the Bronze and Iron Ages. Some were used in the post-Roman period. The fortification usually follows the contours of a hill, consisting of one or more lines of earthworks, with or defensive walls, and external ditches. Hill forts developed in the Late Bronze and Early Iron Age, roughly the start of the first millennium BC, and were in use in many Celtic areas of central and western Europe until the Roman conquest.

The terms "hill fort", "hill-fort" and "hillfort" are all used in the archaeological literature. They all refer to an elevated site with one or more ramparts made of earth, stone and/or wood, with an external ditch. Many small early hill forts were abandoned, with the larger ones being redeveloped at a later date. Some hill forts contain houses.

Similar but smaller and less defendable earthworks are found on the sides of hills. These are known as hill-slope enclosures and may have been animal pens.

They are most common during later periods:

Prehistoric Europe saw a growing population. It has been estimated that in about 5000 BC during the Neolithic between 2 million and 5 million lived in Europe; in the Late Iron Age it had an estimated population of around 15 to 30 million. Outside Greece and Italy, which were more densely populated, the vast majority of settlements in the Iron Age were small, with perhaps no more than 50 inhabitants. Hill forts were the exception, and were the home of up to 1,000 people. With the emergence of oppida in the Late Iron Age, settlements could reach as large as 10,000 inhabitants. As the population increased so did the complexity of prehistoric societies. Around 1100 BC hill forts emerged and in the following centuries spread through Europe. They served a range of purposes and were variously tribal centres, defended places, foci of ritual activity, and places of production.

...
Wikipedia