Castles and coercion: how protection markets reshaped power in post-Roman Europe

When the Roman Empire lost its grip on the West in the fifth century, the change was not simply the collapse of an army or the disappearance of emperors. It was a fundamental rewiring of how power worked across space, according to a new study published in the International Review of Economics. Mingshu Wang, an independent researcher based in Hong Kong, argues that the transformation of post-Roman Europe between roughly 400 and 900 CE is best understood not as the fall of a civilization but as the replacement of one spatial economic system with another: the sophisticated Roman tax state gave way to what the author calls a “protection market,” organized around fortified nodes and the zones of enforceable authority that radiated outward from them.

At the heart of the paper is a simple but powerful analytical concept: the coercion radius. Wang defines this as the area surrounding a fortified place, a walled city, a hillfort, a monastery, a bridgehead, a castrum, an estate complex, or one of Alfred the Great’s burhs, within which the holder of that stronghold could genuinely enforce judgment, compel labor, tax the movement of goods, organize collective defense, and credibly threaten violence. Under the Roman Empire, this kind of coercive reach extended across an entire Mediterranean-spanning geography, held together by a fiscal apparatus that linked land assessment, military provisioning, judicial authority, and long-distance logistics into a single, continuous system of rule. After the western empire contracted, that continuity shattered, and authority had to be rebuilt from defended places outward.

The study is careful to reject the older, dramatic narrative in which Roman civilization simply “fell” and was swept away. Drawing on late Roman law, the letters of Cassiodorus, the histories of Procopius and Gregory of Tours, Visigothic legal codes and church councils, Frankish capitularies, and the Anglo-Saxon Burghal Hidage, Wang shows that nearly all of the institutional furniture of Rome survived the transition. Law codes remained in use, cities persisted as centers of episcopal authority, imperial offices and titles continued to confer prestige, and fiscal memory, the accumulated knowledge of who owed what to whom, endured in both written and customary form. What changed, the paper argues, was not the content of these instruments but the distance at which they could be made to work. Roman forms remained usable; they no longer functioned at continental scale.

This reframing has significant implications for how economists and historians understand the origins of medieval political order. In the Roman tax state, the emperor’s power rested on the ability to assess land, collect taxes at a distance, pay armies, and provision cities through Mediterranean shipping networks. Coercion and revenue flowed through administrative channels that operated independently of any single location. In the post-Roman world, by contrast, no ruler could count on such reach. Instead, power had to be made credible locally. A king, bishop, abbot, or magnate who wanted peasants to deliver rents, laborers to repair walls, merchants to pay tolls, or warriors to serve in defense had to anchor that authority in a physical stronghold from which force could actually be projected and punishment actually delivered.

The protection market that emerged from this constraint was, in Wang’s account, a genuine market in the economic sense. Those who controlled fortified nodes offered something scarce and valuable: security. Those who lived within the coercion radius paid for it with rents, tolls, service obligations, labor dues, transport duties, repair work, and obedience. The exchange was unequal, often coercive, and frequently resented, but it was structured by supply and demand. Peasants and townspeople needed protection from raiders, rival lords, and endemic violence; lords needed resources to maintain walls, garrisons, and followings. The terms of exchange varied by region and period, and the resulting patchwork of obligations became the raw material out of which territorial lordship, and eventually the medieval state, would crystallize.

The paper grounds this argument in a wide body of evidence. In Visigothic Spain, royal law continued to articulate fiscal claims well after the capability to enforce them uniformly had eroded, and church councils reveal the ongoing negotiation between royal authority and local power holders. In Frankish Gaul, Merovingian and Carolingian rulers issued capitularies that presupposed local fortifications and local officials as the practical machinery of governance; the reach of Charlemagne’s order depended on counts, castellans, and monasteries who could make royal commands stick within their own spheres. In Anglo-Saxon England, the Burghal Hidage, the famous tenth-century document listing fortified burhs and the manpower allocated to maintain their walls, offers a nearly quantitative snapshot of the coercion-radius logic: each fortress’s garrison obligation was calculated in proportion to the length of its defenses, a direct fiscal expression of defended-place power.

Archaeological scholarship on Italian cities, Iberian local societies, and the rural landscapes of northwest Europe reinforces the picture. Urban centers contracted, walled themselves, and became episcopal strongholds rather than administrative hubs of empire. Hillforts and estate centers multiplied in regions where Roman administration had once mediated between center and locality. Trade and communications reorganized around shorter, safer circuits, a shift long documented in scholarship on the early medieval economy. Wang synthesizes these strands into a single spatial-economic model: the fragmentation of empire was, above all, a fragmentation of reach, and fortifications were the technology that defined the new unit of reach.

Crucially, the study does not claim that fortifications caused the new order by themselves. Walls and strongholds, Wang emphasizes, made inherited and emerging forms of command defensible, repeatable, and locally credible once the Roman tax state lost its western depth. Roman law, episcopal authority, imperial titles, and fiscal habits were the software of post-Roman power; fortifications were the hardware that allowed that software to run at reduced scale. Without the strongholds, traditional claims to authority would have been empty words. Without the inherited claims, strongholds would have been mere bandit lairs. The combination of the two produced stable, reproducible lordship, and over centuries, the aggregation of coercion radii into larger and larger polities, culminating in the territorial kingdoms of the high Middle Ages.

The paper also engages with the long-running scholarly debate between proponents of continuity and proponents of rupture. Some historians have stressed how much of the Roman world survived the fifth century, pointing to law, language, the church, and aristocratic culture; others have emphasized the material collapse documented in archaeology, from shrinking cities to the end of industrial-scale pottery production. Wang’s framework offers a way to hold both truths simultaneously. In terms of forms and institutions, continuity dominated. In terms of the spatial extent over which those forms could be enforced, rupture was decisive. The coercion radius captures precisely this asymmetry: the same legal and fiscal instruments persisted, but their effective operating distance collapsed from imperial to local scale, and the entire architecture of political economy reorganized around that new constraint.

The findings speak to questions far beyond late antiquity. Economists studying state capacity have long asked why some political systems can project authority across vast territories while others remain confined to enclaves of control. Wang’s analysis suggests that the answer lies partly in the technology of credible commitment: a state exists, in fiscal terms, wherever it can actually collect taxes and enforce judgments, and that capacity is bounded by the physical infrastructure of coercion. The Roman tax state achieved continental reach through roads, fleets, registers, and legions; its successors achieved local reach through walls, garrisons, and personal obligation. The transition between the two systems reshaped not only medieval Europe but also the deep institutional foundations on which European economic development later unfolded.

Published as volume 73, article 30 of the International Review of Economics, the study received on 30 March 2026, accepted on 13 June 2026, and published on 7 July 2026, arrives as part of a growing effort to bring rigorous economic reasoning to the transformation of the Roman world. By translating the rich historiography of late antiquity and the early Middle Ages into the vocabulary of spatial economics, coercion, and market exchange, Wang offers specialists and general readers alike a new lens on one of history’s most consequential transitions. The fall of Rome, on this account, was less an ending than a rescaling, and the medieval castle, monastery, and burh were not relics of a dark age but the load-bearing structures of a new political economy rising from the ground up.