A hillfort 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 Age or Iron Age. Some were used in the post-Roman period. The fortification usually follows the contours of a hill and consists of one or more lines of earthworks, with stockades or defensive walls, and external ditches. Hillforts developed in the Late Bronze and Early Iron Age, roughly the start of the first millennium BC, and were used in many Celtic areas of central and western Europe until the Roman conquest.

The spellings "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 hillforts were abandoned, with the larger ones being redeveloped at a later date. Some hillforts 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.

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. Hillforts 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.[4] As the population increased so did the complexity of prehistoric societies. Around 1100 BC hillforts 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.[5]

During the Hallstatt C period, hillforts became the dominant settlement type in the west of Hungary.[6] Julius Caesar described the large late Iron Age hillforts he encountered during his campaigns in Gaul as oppida. By this time the larger ones had become more like cities than fortresses and many were assimilated as Roman towns.

Hillforts were frequently occupied by conquering armies, but on other occasions the forts were destroyed, the local people forcibly evicted, and the forts left derelict. For example, Solsbury Hill was sacked and deserted during the Belgic invasions of southern Britain in the 1st century BC. Abandoned forts were sometimes reoccupied and refortified under renewed threat of foreign invasion, such as the Dukes' Wars in Lithuania, and the successive invasions of Britain by Romans, Saxons and Vikings.

Excavations at hillforts in the first half of the 20th century focussed on the defenses, based on the assumption that hillforts were primarily developed for military purposes. The exception to this trend began in the 1930s with a series of excavations undertaken by Mortimer Wheeler at Maiden Castle, Dorset. From 1960 onwards, archaeologists shifted their attention to the interior of hillforts, re-examining their function.[7] Currently, post-processual archaeologists regard hillforts as symbols of wealth and power.[8] Michael Avery has stated the traditional view of hillforts by saying, "The ultimate defensive weapon of European prehistory was the hillfort of the first millennium B.C.".[9] By contrast, Professor Ronald Hutton wrote in the English Heritage Members Magazine in March 2020 "It now seems that they were assembly places where farming families would meet seasonally..."

Beyond the simple definition of hillfort, there is a wide variation in types and periods from the Bronze Age to the Middle Ages. Here are some considerations of general appearance and topology, which can be assessed without archaeological excavation:

Some forts were also settlements, while others were only occupied seasonally, or in times of strife. Archaeological excavation reveals more about the dates of occupation and modes of use. Typical features for excavation include:

The reason for the emergence of hillforts in Britain, and their purpose, has been a subject of debate. It has been argued that they could have been military sites constructed in response to invasion from continental Europe, sites built by invaders, or a military reaction to social tensions caused by an increasing population and consequent pressure on agriculture. The dominant view since the 1960s has been that the increasing use of iron led to social changes in Britain. Deposits of iron ore were located in different places to the tin and copper ore necessary to make bronze, and as a result trading patterns shifted and the old elites lost their economic and social status. Power passed into the hands of a new group of people.[10] Archaeologist Barry Cunliffe believes that population increase still played a role and has stated "[the forts] provided defensive possibilities for the community at those times when the stress [of an increasing population] burst out into open warfare. But I wouldn't see them as having been built because there was a state of war. They would be functional as defensive strongholds when there were tensions and undoubtedly some of them were attacked and destroyed, but this was not the only, or even the most significant, factor in their construction".[11]

Hillforts in Britain are known from the Bronze Age, but the great period of hillfort construction was during the Iron Age, between 700 BC and the Roman conquest of Britain in 43 AD. The Romans occupied some forts, such as the military garrison at Hod Hill, and the temple at Brean Down, but others were destroyed and abandoned. Partially articulated remains of between 28 and 40 men, women and children at Cadbury Castle were thought by the excavator[12] to implicate the Cadbury population in a revolt in the 70s AD (roughly contemporary with that of Boudicca in the East of England), although this has been questioned by subsequent researchers.[13][14] However, the presence of barracks on the hilltop in the decades following the conquest suggest an ongoing struggle to suppress local dissent.

Maiden Castle in Dorset is the largest hillfort in England. Where Roman influence was less strong, such as uninvaded Ireland and unsubdued northern Scotland, hillforts were still built and used for several more centuries.

There are over 2,000 Iron Age hillforts known in Britain of which nearly 600 are in Wales.[15] Danebury in Hampshire, is the most thoroughly investigated Iron Age hillfort in Britain, as well as the most extensively published.[16]

Cadbury Castle, Somerset is the largest amongst forts reoccupied following the end of Roman rule, to defend against pirate raids, and the Anglo-Saxon invasions. The cemetery outside Poundbury Hill contains east-facing Christian burials of the 4th century CE. In Wales, the hillfort at Dinas Powys was a late Iron Age hillfort reoccupied from the 5th-6th centuries CE;[17] similarly at Castell Dinas Brân a hillfort of c.600 BCE was reused in the Middle Ages, with a stone castle built there in the 13th century CE.[18]

Some Iron Age hillforts were also incorporated into medieval frontier earthworks. For example Offa's Dyke, a linear earthwork generally dated to the 9th century CE, makes use of the west and south-west ramparts of Llanymynech hillfort.[19] Similarly the hillfort at Old Oswestry was incorporated into the early medieval Wat's Dyke.[20] The Wansdyke was a new linear earthwork connected to the existing hillfort at Maes Knoll, which defined the Celtic-Saxon border in south-west England during the period 577–652 CE.

Some hillforts were re-occupied by the Anglo-Saxons during the period of Viking raids. King Alfred established a network of coastal hillforts and lookout posts in Wessex, linked by a Herepath, or military road, which enabled his armies to cover Viking movements at sea. For example, see Daw's Castle and Battle of Cynwit.

It has been suggested on reasonable evidence that many so-called hillforts were just used to pen in cattle, horses, or other domesticated animals. The large sprawling examples at Bindon Hill and Bathampton Down are more than 50 acres (20 ha). Even those that were defensive settlements in the Iron Age were sometimes used for corralling animals in later periods. For example, see Coney's Castle, Dolebury Warren and Pilsdon Pen. However, it is difficult to prove that people definitely did not dwell there, as lack of evidence is not proof of absence.

The Hallstatt culture and La Tène culture originated in what is now southern Germany, Switzerland, Austria, Slovakia and the Czech Republic. However, hillforts were built also in Poland and further east, until the Middle Ages.

The predominant form of rampart construction is pfostenschlitzmauer, or Kelheim-style.

During the period of Late Antiquity or Migration Period a large number of hilltop settlements were established both on the Roman imperial territory and on Germanic soil. However, the term embraces a wide range of very different settlements in high locations. At least a few of the Germanic settlements were protected by fortifications. Unlike the Romans, however, the Germanii did not use mortar at that time for their construction. Among the best known hill settlements in Germany are the Runder Berg near Bad Urach and the Gelbe Bürg near Dittenheim. Even in areas that were remote from the Roman Empire, such as southern Sweden, numerous hillfort sites of this period have been found.

In Galicia, Asturias, Cantabria, Basque Country, province of Ávila and Northern Portugal a castro is a fortified pre-Roman Iron Age village, usually located on a hill or some naturally easy defendable place.[21] The larger hillforts are also called citanias, cividades or cidás (English: cities). They were located on hilltops, which allowed tactical control over the surrounding countryside and provided natural defences. They usually had access to a spring or small creek to provide water; some even had large reservoirs to use during sieges. Typically, a castro had one to five stone and earth walls, which complemented the natural defences of the hill. The buildings inside, most of them circular in shape, some rectangular, were about 3.5–15 m (11–49 ft) long; they were made out of stone with thatch roofs resting on a wood column in the centre of the building. In the major oppida there were regular streets, suggesting some form of central organization. Castros vary in area from less than a hectare to some 50 hectare ones, and most were abandoned after the Roman conquest of the territory.

Many castros were already established during the Atlantic Bronze Age period, pre-dating Hallstatt culture.

Many of the megaliths from the Bronze Age such as menhirs and dolmens, which are frequently located near the castros, also pre-date the Celts in Portugal, Asturias and Galicia as well as in Atlantic France, Britain and Ireland. These megaliths were probably reused in syncretic rituals by the Celtic Druids.

The Celtiberian people occupied an inland region in central northern Spain, straddling the upper valleys of the Ebro, Douro and Tajo. They built hillforts, fortified hilltop towns and oppida, including Numantia.

The Estonian word for hillfort is linnamägi (plural linnamäed), meaning hillfort or hillburgh. There are several hundred hillforts or presumed ancient hillfort sites all over Estonia. Some of them, like Toompea in Tallinn or Toomemägi in Tartu, are governance centres used since ancient times up until today. Some others, like Varbola are historical sites nowadays.

Most likely the Estonian hillforts were in pre-Christian times administrative, economic and military centres of Estonian tribes. Although some of them were probably used only during times of crisis and stood empty in peacetime (for example Soontagana in Koonga parish, Pärnu county).

The Finnish word for hillfort is linnavuori (plural linnavuoret), meaning fort hill or castle hill, or alternatively muinaislinna meaning ancient fort, as opposed to bare linna which refers to medieval or later fortifications.

One special feature about the Finnish hillforts that while most of them are located these days within some distance from the sea, but earlier many of the forts were located by the sea, due to post-glacial rebound.

Finland has around 100 hillforts verified by excavations, and about 200 more suspected sites. The largest hillfort in Finland is the Rapola Castle, another notable one is the Old Castle of Lieto.

Bronze Age and Iron Age hillforts are widely found in Ireland. They are large circular structures between 1 and 40 acres (most commonly 5–10 acres) in size, enclosed by a stone wall or earthen rampart or both. These would have been important tribal centres where the chief or king of the area would live with his extended family and support themselves by farming and renting cattle to their underlings.

There are around 40 known hillforts in Ireland.[22] About 12 are multivallate as distinguished by multiple ramparts, or a large counterscarp (outer bank). The imposing example at Mooghaun is defended by multiple stone walls.

One must be careful to not confuse a hill-fort with a 'ringfort' - a medieval settlement - a common archaeological feature across the whole island of Ireland, of which over 40,000 examples are known;[23] one source claims there may be 10,000 undiscovered ringforts.[24]

Some hillforts have cairns inside their boundaries and there are many speculations about this phenomenon, the theories range from being a strange cult religion to just coincidence the same kind of area as they both like (hill tops with commanding views of the local vicinity), the excavation at Freestone Hill in County Kilkenny has shown that there was indeed a ditch cut out around the cairn, evidence that they had respect for the feature no matter what they decided to believe about it.

The Latvian word for hillfort is pilskalns (plural: pilskalni), from pils (castle) and kalns (hill).

Hillforts in Latvia offered not only military and administrative functions but they were also cultural and economic centres of some regions. Latvian hillforts generally were a part of a complex consisting of the main fortress, the settlement around it, one or more burial fields and nearby ritual sites. The first hillforts in Latvia, such as Daugmale hillfort, appeared during the Bronze Age.[25] Some were continuously inhabited until the late Iron Age.[26][27]

During the Roman Iron Age, some of the Latvian hillforts (like Ķivutkalns) were abandoned or became sparsely populated. A new period in hillfort development started during the 5th-8th centuries AD, when many new hillforts appeared, in most cases, along the main trades routes - rivers. During the 10th-11th centuries, some of the hillforts became military fortresses with strong fortifications (like hillforts in Tērvete, Talsi, Mežotne). Some of them are considered important political centres of the local peoples, who in this period were subjects of serious social political changes. That period was known for unrest and military activities, as well as power struggles between local aristocracy. Most of the Latvian hillforts were destroyed or abandoned during the Livonian Crusade in the 13th century, but some were still used in the 14th century. In total, there are about 470 hillforts in Latvia.[26][27]

The Lithuanian word for hillfort is piliakalnis (plural piliakalniai), from pilis (=castle) and kalnas (=mountain, hill).

Lithuania has hillforts dating from the Bronze Age in the 1st millennium BC. The earliest examples in present-day Lithuania are found in the east of the country. Most of these forts were built or expanded between the fifth and fifteenth centuries, when they were used in the Dukes' Wars, and against the invasion of Teutonic Knights from the west. Most forts were located on the banks of a river, or a confluence where two rivers met. These fortifications were typically wooden, although some had additional stone or brick walls. The hill was usually sculpted for defensive purposes, with the top flattened and the natural slopes made steeper for defence.

During the early years of Grand Duchy of Lithuania piliakalniai played a major role in conflicts with the Livonian Order and the Teutonic Knights. During this period the number of piliakalniai in use decreased, but those that remained had stronger fortifications. Two main defence lines developed: one along the Neman River (against the Teutonic Order) and another along the border with Livonia. Two other lines started to form, but did not fully develop. One was to protect Vilnius, the capital, and the other line in Samogitia, was a major target for both orders. This territory separated the two Orders and prevented joint action between them and Pagan Lithuania.

According to the Lietuvos piliakalnių atlasas (English: Atlas of Piliakalniai in Lithuania), there were 826 piliakalniai in Lithuania. Some researchers present a total number of 840 known piliakalnis in 2007; the number is likely to increase as even more of them are discovered every year. Most piliakalniai are located near rivers and are endangered by erosion: many have partly collapsed as the flooded river has washed out the base of the hill. Now around 80 percent of piliakalniai are covered by forests and are hardly accessible to visitors.

In Scandinavia and northern Russia, hillforts are fortifications from the Iron Age which may have had several functions. They are usually located on the crests of hills and mountains making use of precipices and marshes which worked as natural defences. The crests' more accessible parts were defended with walls of stone and outer walls in the slopes beneath are common. Round and closed, so-called ring forts are common even on flat ground. The walls often have remaining parts of stone, which were probably the support of pales. They often have well delineated gateways, the gates of which were probably of wood. Hillforts with strong walls are often located beside old trade routes and have an offensive character, whereas others are reclusive and were weakly fortified, probably only for hiding during raids.

Many forts, located centrally in densely populated areas, were permanently settled strongholds and can show traces of settlements both inside and outside. Older place names containing the element sten/stein were usually hillforts.

In Sweden, there are 1,100 known hillforts with a strong concentration on the northern west coast and in eastern Svealand. In Södermanland there are 300, in Uppland 150, Östergötland 130, and 90 to 100 in each of Bohuslän and Gotland. Norway has about 400 hillforts, Denmark has 26.