Rethinking Iron Age Britain: Hillforts, Oppida, and the Origins of Urban Assembly

by Mary Harrsch © 2026

This morning I was listening to a podcast about Iron Age Britain and was surprised to learn hillforts and oppida are two very distinct architectural developments. I had always thought about these terms as interchangeable but the functional and morphological differences between the hillfort and the oppidum are far more significant than casual usage of these terms tends to suggest.

A reconstruction of the Basel oppidum  in Switzerland, 80 BCE  (CCO 1.0)

My research revealed the hillfort, dominant across the Middle Iron Age (c. 400–100 BCE), is characterized by its elevated topographic position, a single continuous defensive circuit of timber-laced ramparts and ditches, and a relatively homogeneous internal layout of roundhouses and grain storage features. These ramparts typically employed a box or fach construction — a wooden latticework filled with rubble and fronted by stone or compacted clay — effective, but vulnerable over time to fire and decay. Sites such as Maiden Castle (Dorset) and Danebury (Hampshire) exemplify this form. Their primary logic was defensive consolidation and the visible expression of communal territorial authority at a local or clan level: a single, unbroken perimeter through which no entry was possible without passing a defended gate.

The oppidum (pl. oppida), emerging in the Late Iron Age from roughly 150 BCE onward, represents a qualitatively different phenomenon in nearly every dimension — including its fortification technology. Rather than the timber-laced rampart of the hillfort tradition, oppida frequently employed the murus Gallicus ("Gallic wall"), the most sophisticated defensive construction of the Iron Age. As described by Julius Caesar, this technique involved horizontal timber beams laid perpendicular to the wall face, pinned by iron nails, and sandwiched between a dressed drystone facing and a rubble core. The result was a structure that was substantially more fire-resistant and structurally robust than its predecessors — and the quantity of iron required for its construction was itself a conspicuous signal of accumulated wealth and organizational capacity.

Equally significant is the strategic philosophy underlying oppidum defenses. Where the hillfort enclosed a single hilltop within a neat, unbroken ring, the oppidum deployed multiple, discontinuous linear earthworks to control movement across a much larger and more irregular landscape. These are perhaps best described as enclosed but not fully encircled — defended by a discontinuous system rather than a single perimeter. Camulodunum (Colchester) illustrates this well: situated on a low plateau rather than a prominent peak, three sides of the site benefit from natural protection afforded by marshland and the River Colne, while the landward approach is blocked by a series of massive linear earthworks — Gryme's Dyke and Berechurch Dyke among them — stretching for several miles. Large areas within this defensive zone remain open, and the logic of the system is the control of movement through defined corridors rather than the creation of a fortress.

Internally, oppida reflect an emerging proto-urbanism that stands in sharp contrast to the hillfort's relative homogeneity: zoned quarters for residential occupation, specialist craft production (including on-site coin minting), and — of particular interpretive interest — large open areas understood as deliberative or assembly spaces. It is this last feature that marks the oppidum as a central place in a genuinely political sense: a site where regional elites, tribal councils, and wider populations convened for governance, dispute resolution, and ceremonial life.

The transition between these two settlement forms tracks closely with intensifying continental contact, the influence of Gaulish oppida described by Caesar, and the growth of long-distance exchange networks linking Britain to the wider Mediterranean world. It is no coincidence that Roman administrators later adopted many of these same sites as their provincial and regional capitals.

For those who want to explore this further, The Ancients podcast covered this material in an accessible and well-researched episode — well worth a listen.

Image: Basel oppidum reconstruction, Switzerland, c. 80 BCE (CC0 1.0)

#IronAgeBritain #Archaeology #Prehistory #Hillforts #Oppida #LateIronAge #MurusGallicus

See less

If you enjoyed this post, never miss out on future posts by following me by email!

Beyond the Antonine Wall: Rome’s Forgotten Frontier in the Highlands

 by Mary Harrsch © 2026

I was listening to "The Ancients" podcast on Amazon Music while I exercised this morning and the guest historian mentioned the remains of over 150 Roman forts have been identified in Scotland. I researched this a little further and discovered while over 150 Roman forts have been identified, the total number of Roman sites in Scotland, including temporary camps and other installations, is over 330. The concentration of Roman remains in Scotland is the direct result of two major, but ultimately unsuccessful, military campaigns to conquer the tribes of the north, known to the Romans as the Caledonii 

Reconstruction of a Roman fortlet from Gask Ridge 1st century CE courtesy of Wikimedia Commons contributor Veleius, colorized by the author

These campaigns left behind a complex landscape of military architecture, which can be grouped into three main categories:

• The Forts (Over 150 Identified): These permanent bases were garrisoned by auxiliary soldiers to control the local population and patrol the frontier. They varied in size from "slight" fortlets holding 50 men to large military complexes that could house a 500-strong cavalry regiment .
• The Walls: The Romans built two famous linear barriers. Hadrian's Wall (started 122 AD) was the empire's northern boundary. Later, the Antonine Wall (started 142 AD) was pushed further north, running between the Firth of Forth and the Firth of Clyde, effectively cutting Scotland in two.
• The Temporary Camps: These were marching camps built of turf and earthworks, thrown up by legionaries at the end of each day's march. Scotland is one of the best places in the former Roman Empire to study these camps because they are so well-preserved and numerous

Before the construction of the Antonine Wall, Flavian forces constructed what has become known as the Gask Ridge Frontier. This line of forts and watchtowers between Dunblane in Stirling and the River Tay in Perthshire was constructed in the 70s or 80s CE, predating the more famous German frontiers, making it a prototype for how Rome controlled its borders. The fortifications run along a 10-16 mile (16-37 km) ridge of high ground that naturally separates the Scottish Lowlands from the Highlands. The name itself comes from the Scottish Gaelic word gasg, meaning a "projecting tail or strip of land.”

The Main Forts (from south to north):

• Camelon
• Doune
• Glenbank (fortlet)
• Ardoch (a major fort and likely supply depot)
• Kaims Castle (fortlet)
• Strageath
• Bertha (near modern Perth)
• Cargill (fort and fortlet)
• Inchtuthil (the massive legionary fortress—more on this below)
• Cardean
• Stracathro

The Watchtowers

A distinctive feature of the Gask Ridge is its series of timber watchtowers (signal stations). One of the best-preserved examples is Muir o' Fauld, where you can still see the circular mound, ditch, and outer bank that surrounded the original timber tower.

Forts like Drumquhassle, Bochastle, Dalginross, and Fendoch, known as the Glenblocker Forts, were positioned at the exits of Highland glens. Older scholarship saw these as a separate "staged withdrawal" line, but modern research views them as an integrated part of the same frontier system, controlling access to the major valleys that lead into the Highlands.

The largest installation in the system, Inchtuthil was a full-scale legionary fortress built to house the Legio XX Valeria Victrix. Its size and strategic position (with access into Braemar and beyond) made it the major deterrent and potential springboard for further invasions northward.

Based on the historical account of the Roman governor Gnaeus Julius Agricola (father-in-law of the historian Tacitus) and archaeological evidence, Initial construction began under governor Petilius Cerealis in 70 CE. The fortifications were subsequently used during Agricola’s campaigns in Scotland from 79-80 CE and during 84 CE when Agricola defeated Calgacus at the Battle of Mons Graupius. Abandonment and withdrawal south did not occur until 86-87/88 CE. Archaeological excavations have revealed that many of the forts and watchtowers were rebuilt, sometimes twice, without evidence of destruction by warfare. This suggests the Romans actually stayed in the area for up to 15 years.

However, during the Antonine Period (c. AD 142-165): Forts such as Ardoch, Strageath, Bertha, and Dalginross show evidence of reuse, contemporary with the Antonine Wall further south. The area was again under Roman occupation during Emperor Septimius Severus's campaigns (208-211). This time, the focus was on the legionary fortress at Carpow, downstream from modern Perth.

 Dr. David Woolliscroft, Director of the Roman Gask Project, has proposed instead of a frontier built solely for conquest and defense against hostile Caledonian tribes, Gask Ridge may have served a protective and trading function:

1. Defense of harbors: The system guarded strategically important harbors at the Firths of Tay and Forth, critical for Roman supply lines.
2. Protection of trading partners: Rather than fighting all locals, the Romans appear to have established trading relationships with farming communities, exchanging Roman goods (wine, olive oil) for local products (beer, mutton). The Gask Ridge may have been built to protect these allied farmers from cattle raiders sweeping down from the Highlands.
3. Supervision, not blockade: The Glenblocker forts lacked the manpower to stop a major invasion, but they were well-positioned to monitor and control movement through the glens.

Archaeological evidence supports this: native Iron Age settlements show no signs of destruction, and farming appears to have flourished during the Roman presence. 

If you enjoyed this post, never miss out on future posts by following me by email!