The findings of the geophysical surveys at Aggersborg and the resultant wider reassessment of the site, including the evidence from the early excavations, have contributed a significantly more precise understanding of the settlement that preceded the construction of the great ring-fortress. In so doing, they also offer a basis for a new understanding as to why this particular site was chosen as the location for an exceptional military building work in the late 10th century. The results demonstrate that the ring fortress replaced a site of particular importance, but one tuned to a very different social strategy and organisation.
The significance of the pre-fortress settlement is brought out by a number of factors. The extraordinary number of SFBs indicates that the settlement was characterised by some specific activity not typical of rural settlements. The construction of SFBs over a period of more than 200 years – even after the abandonment of the fortress – means that the activities associated with them were a firmly entrenched characteristic of the site. The occurrence of these buildings within a settlement, in which one particularly large house stands out, supports the proposition that they were embedded in asymmetrical social relations – presumably serving as shelters and workshops for subordinates, who were obliged to render labour service, including textile working, to a lord. The location of a busy elite site at an important crossing for land-traffic, a place where local traffic convened, suggests that activities might well have included periodic gatherings, as was almost certainly the case in the 11th century when the settlement is attested as a crown estate. The existence of contemporary sites with a similar appearance in at least two other locations along the eastern Limfjord – Bejsebakken and Sebbersund – implies that Aggersborg's significance was mainly local. Nevertheless, the strategic position overlooking the best sheltered waterway between the North Sea and the Baltic, and commanding a potentially crucial post in a warning system, suggests that it would have been an essential asset for any power seeking to control the rich Limfjord region in a society tuned to maritime interaction. It is unlikely that such a position was left at will in the hands of a random local magnate. Despite a general scarcity of any conspicuous signs of wealth among the finds from Aggersborg (pace the gold armring from house D), the situation of the site allows us to assume that the lord would depend on benign relations with wider, extra-regional powers.
According to the interpretation advanced here, the pre-fortress settlement would thus have convened a substantial number of people under the auspices of the notably large main farm. It was likely to have been a centre for local traffic for subordinates from neighbouring settlements, as well as a centre for a wider regional traffic due to the crossing point, and arguably a site for periodic assemblies. The confluence of regional traffic would also have made Aggersborg a convenient, and visible, destination for extra-regional travellers seeking opportunities for exchange, a circumstance that may be reflected in the quantity of non-local objects: periodic assemblies in particular would have presented good and safe market opportunities for non-local visitors. Even without an organised warning system, the exceptional view along the access routes of the fjord would have made Aggersborg a particularly safe location against naval attacks. Finally, the position at the Limfjord, an artery for long-distance travel, made the place potentially a stepping-stone for even wider traffic.
This characterisation of the pre-fortress settlement suggests compelling reasons why this place was selected as the site for the ring fortress. It also suggests that some of the function that the fortress was intended to perform could have been inherited from the earlier settlement. In as much as the power of the lord residing in Aggersborg's house D demanded good relations to a supra-regional power, it almost certainly assumed a durable bond of loyalty to a king. A likely model for this relationship may be found in that described in early Norwegian laws by the office as a royal lensmann ('sheriff'), and in Danish medieval laws that of a kongsbryde ('king's estate manager'), or later ombudsmand (Skrubbeltrang 1957; Andersen 2011, 199). The lensmann, kongsbryde or ombudsmand was characterised as an estate manager who had been commissioned to collect Crown dues within his county; who enjoyed maintenance at a manor recognised as a centre for the county as well as other individually agreed privileges; and whose duty, in return, was to act as the king's legal representative and to host the king and his retinue during their travels. If the pre-fortress settlement was some such estate centre, the fortress would in important respects have been a continuation of the previous institution by different means.
The observations presented here offer a case for a reassessment of Aggersborg ring-fortress, and potentially other members of the Trelleborg group of fortresses, as being more closely aligned to the political and tactical use of fortified places elsewhere in Early Medieval Europe than their conventional description as 'Viking fortresses' would suggest (cf. critique by Wilson 1978; Williams 2008). As a system of a newly constructed fortified place, which was to replace previously existing county manors as centres for royal authority, the Trelleborg fortresses echo the Wessex burghal system of early 10th-century Anglo-Saxon England, as outlined in the burghal hidage. This document, which was probably drawn up in AD 914, lists the services and fees that the free population in the kingdom of King Alfred's successors was required to provide for the maintenance, supply and defence of regional fortresses (Hill and Rumble 1996). The burghal system was established at the end of the 9th century in response to the Great Heathen Army of Scandinavian Vikings, active in England in the period AD 865-74 (Williams 2008, 198f; cf. also Abels 1997; Haslam 2005). The Vikings spurred a shift in the organisation of Anglo-Saxon royal power towards fortified centres where income and taxes could be collected and guarded by a permanent garrison with their families, craftsmen and servants. Here a large army could be accommodated and supplied and, equally importantly, a hostile army could be prevented from doing the same.
A similar system was instituted elsewhere in the 10th century, in the Netherlands by Count Baldwin II, similarly against Vikings, and in Germany by Heinrich I of Saxony against the Hungarian armies. According to Heinrich's famous legislation, the Burgenordnung of AD 926, one in every nine warriors was obliged to man a number of newly constructed fortresses, to which the other eight should bring supplies. Heinrich also gave orders for assemblies and feasts to be held in these fortresses (Büttner 1956).
The parallels between these systems and the potential use of the Danish ring forts include the spacious size of the fortresses, their supra-regional organisation, their location on important communication lines, and their origin in response to confrontation with foreign armies. If, as argued here, Aggersborg replaced a site that already acted as a county manor and centre for royal authority, this adds a further key element to the analogy. It would suggest that the dues and services previously paid to this centre, and to which the large number of SFBs has been argued here to be directly related, were also an important element in the strategy devised for the construction and maintenance of the short-lived ring-fortress. The Trelleborg fortresses emerge as less enigmatic, but no less impressive, expressions of power. The policy they embody is, in fact, much in line with those pursued by powerful rulers in challenging circumstances elsewhere in 10th-century Europe. As a case for understanding the political and tactical role of fortified places in Early Medieval Europe, they are far more revealing than the traditional Viking image – and their justly celebrated architecture – have led us to believe.