Reports on Medieval Castle Building between Architecture, Aesthetics, and Archaeological Interest

We do not know many architects and master builders from the Middle Ages, compared to the vast number of buildings that have survived to this day. Particularly when it comes to castle construction, only a few names are known to us. One example from the British Isles is James of St. George, who is known to have been in the service of Edward I. The castles he designed, such as Harlech, Beaumaris, and Conwy, bear his characteristic signature. We are fairly well-informed about his work for the English king, but we lack an overview of his life, his educational background, and his influences.

However, we can study one castle architect thanks to a relatively good source base: Bishop Benno II of Osnabrück. He is known as the architect of the castles in the Harz region, which Henry IV had built. I have already discussed this castle-building activity elsewhere.

Henry IV tried to reopen the old royal landscape around the Harz region through the construction of castles. He built several castles around the Harz (e.g., Wigantenstein, Moseburg, Sachsenstein, Spatenberg, Heimburg, Vockenrode, and Asenberg), but the Harzburg holds particular significance. It bore the name „Harz“ and aimed to dominate the entire landscape.

„He fortified it [the Harzburg] outwardly with a strong wall, towers, and gates, adorned it inwardly with truly royal buildings, established a chapter there, and gathered such rich furnishings and a sizable clergy from all over that, thanks to its entire arrangement, it equaled some bishoprics and even surpassed others.“

With the castles around the Harz, Benno certainly introduced something new. Previously, Saxon ring walls had dominated the landscape, but he moved the new fortifications to higher ground. Stone masonry became the dominant construction technique, and towers and gates took on forms unfamiliar to the landscape north of the Harz. The local population perceived these castles as foreign and threatening, a sentiment that fueled the conflict between the Saxons and the king. The siege and plundering of the castles, particularly the Harzburg, echoed widely in the written sources and caused a stir throughout the empire.

But who was the architect of these castles, and what can we say about him? Information is provided by the vita written by Abbot Norbert of Iburg shortly after the death of the Osnabrück bishop. Norbert was well-acquainted with Benno and had access to a wealth of information about his life. In his preface, he states that he wrote his text as best he could remember.

Benno came from Swabia, from the town of Löhningen. His parents were not of noble birth, Norbert tells us, but their status exceeded that of common folk—we may view them as representatives of the ministerial class. The young Benno received his first education in Strasbourg, where he had already shown great talent.

The second stage of his education took him to the Reichenau to Hermann the Lame. Norbert writes of this:

„As a youth, he was eager to seek out Hermann the Lame, who enjoyed an excellent reputation in the liberal arts at the time, and whose outstanding works are still preserved today.“

This quote only faintly reflects Hermann’s fame, who was a major authority in mathematics and authored key texts of his time. Not only was he a master in making and using astrolabes, but he also excelled in clocks, musical instruments, and mechanical devices, as another chronicler writes. Despite his speech impediment, Hermann was an eloquent and diligent (eloquens et sedulus) teacher.

After his time at the Reichenau monastery, Benno embarked on a journey that took him as far as Jerusalem—a journey that must have greatly enriched his experience.

In Speyer, Benno first came into contact with the king. Since Conrad II, Speyer had experienced an unprecedented rise. The formerly rather poor city was to receive the largest cathedral in the West. After his father’s death in 1039, Henry III devoted much attention to the construction of the Speyer Cathedral. The city’s boom attracted many interesting personalities of the time, including Benno. Here, he gained his first experience on major construction sites.

In the retinue of Henry III, Benno later moved to Saxony. The imperial palace of Goslar, which was to replace Werla as the central royal site, was being built there. Benno became a teacher at the cathedral school in Hildesheim, where, according to Norbert, he had time to dedicate himself to his studies without external obligations. The well-equipped library, since the time of Bernward of Hildesheim, likely opened up a vast world of knowledge to him, which would have a significant influence later on—something I will return to later.

During a campaign to Hungary, which Benno accompanied in the retinue of the Bishop of Hildesheim, he was able to prove his organizational talent. Even when the king himself suffered from hunger during the campaign, Norbert reports, the Hildesheim contingent always had enough provisions. With incredible ingenuity (incredibili arte), Benno managed to secure enough bread. As a result, the bishop appointed him provost of the Hildesheim diocese in recognition of his merits. Additionally, he became archpriest at the court in Goslar. He was even entrusted with the administration of the royal court itself—he had, according to his biographer, fortified it with discretion, strictness, and justice.

For a short time, Benno moved to the diocese of Cologne to manage its secular administration. However, after just two years, he returned to Hildesheim. Under Henry IV, Benno eventually relocated entirely to Goslar, where he presided over both ecclesiastical and secular courts. His relationship with the very young king was excellent, according to his biographer. The king wanted to keep him close: having often tested Benno’s good qualities, Henry intended him to become a bishop somewhere where the king could easily reach him. But the time for a bishopric had not yet come; Benno was needed at the royal court in Goslar. All internal affairs at the court (infra palacium) were conducted according to his advice. Let us hear from Norbert again:

„Moreover, he (Benno) was an excellent master builder, a genius in the field of stone construction, which was one of the reasons why the aforementioned king held him in inseparable friendship. For even then, the first signs of the Saxon war were sprouting, a conflict that still devastates the world with sorrow after so many years. The king, aware of this, began to fortify all of Saxony with new and strong castles and tried to forestall the defection of the disloyal by strengthening the land. The swift and careful execution of this project was entrusted by the king to Lord Benno, knowing that no one else would approach this task with greater reliability and diligence.“

This is a remarkable testimony to the authorship of the new castles around the Harz, whose construction was based on Benno’s knowledge and considerations. Norbert further explains:

„One could already see his masterful skill in Hildesheim, where he was then provost, and where under the then Bishop Hezilo of blessed memory, many notable buildings were erected under Benno’s leadership, as is well known. […] His fame spread, and foreign rulers and lords began to seek his acquaintance.“

But Benno was also an intellectual architect, an architect in the modern sense, as becomes clear elsewhere.

„In addition to the knowledge and experience that made him perfectly suited for estate management at the highest level—listing them all would take too long—he also possessed remarkable expertise in more everyday matters. In agriculture, that is, in the construction of buildings, cattle breeding, crop cultivation, and all other aspects of rural culture, he far surpassed all others. Almost no one had more skill and greater success in these areas. Yet it is certain that he did not acquire this knowledge through practice but through theoretical study.“

He was a well-read man who did not gain his practical knowledge through hands-on experience but through study. He drew this knowledge from the ancient books available in monastery and cathedral libraries, as well as at court. Through library catalogs and codicological research, we can get a sense of which works were available to him. I would now like to introduce three of them and show their relevance to high medieval castle construction.

The first work from which Benno acquired the necessary knowledge for building is an ancient collection of writings.

The text „Corpus Agrimensorum Romanorum“ is a collection of writings dealing with Roman land surveying (agrimensura). The texts compiled in it date from the first to the fifth century. The oldest surviving manuscript is the Codex Arcerianus, which is now housed in the Herzog August Library in Wolfenbüttel and dates back to the sixth century. These writings were of utmost importance for the administration and division of land in the Roman world, especially in the colonies and provinces. The agrimensores, responsible for surveying and marking boundaries, had the task of measuring land precisely, establishing boundaries, and settling land disputes. The Corpus, for these reasons, addresses the following topics in detail:

First, types of land: It describes various types of land, including agricultural land, public land, land defined by natural boundaries like rivers or mountains, and land brought into focus by legal disputes.

Furthermore, on land surveying methods: Detailed instructions on the measurement of land, particularly the use of „limites“ (boundary lines), „cardines“ (north-south lines), and „decumani“ (east-west lines), which served as the framework for dividing land. These surveys were based on the knowledge of the Etruscans and developed into a systematic approach to managing agricultural areas.

Third, on boundary disputes and legal issues: Another significant portion of the texts deals with resolving boundary disputes between landowners. It lists 15 different types of legal disputes, ranging from the location of boundary stones to questions of property rights and the use of water resources.

Fourth, on public and private land: It describes how public land is distinguished from private land, as well as the role of Roman colonies and provinces in managing these areas.

Finally, on technical terms and tools: The text also contains many technical terms and describes the tools and procedures used by the agrimensores (land surveyors). It explains how measurements were conducted and how the results were documented.

Benno likely encountered this text, particularly at the Reichenau Monastery under Hermann the Lame. Hermann placed great emphasis on geometry and precise calculations, passing this on to his students—we have already mentioned this. Benno could have tested his knowledge of land surveying during the expansion of the city of Speyer and on construction sites in Hildesheim. However, it became particularly important to him during the construction of the castles in the Harz. The Harzburg, which was supplied by a long-distance water pipeline, was built with this theoretical knowledge in mind.

The second book that conveyed architectural knowledge is Isidore of Seville’s encyclopedia.

In all matters concerning medieval Latin, Isidore of Seville’s encyclopedia was authoritative. His most significant work is his 20-book Etymologiae or Etymologiarum sive originum libri XX. Although originally dedicated to the Visigothic king Sisebut, who died in 621, the work remained unfinished at Isidore’s death in 636. It was his disciple Braulio who later arranged and published the encyclopedia. Isidore’s work shaped the knowledge of his time and beyond, up to the early modern period. The first three books, dealing with the literary trivium (grammar, rhetoric, dialectic) and the mathematical quadrivium (arithmetic, geometry, astronomy, music), were especially influential as they formed the foundation of higher education in the Middle Ages.

Isidore’s geographical and cosmographical sections, particularly in books XIII, XIV, and XV, had a profound impact on medieval cartography. His descriptions of oceans, seas, mountains, and cities shaped the understanding of the world at that time. Another noteworthy chapter of his encyclopedia, De Medicina, addresses philosophy and medicine, which he referred to as „secunda philosophia“—disciplines encompassing the entirety of human concerns. Here, Isidore provides a detailed account of the history and tasks of medicine, emphasizing that a physician should be trained in all subjects of the trivium and quadrivium, although medicine itself was not considered one of the liberal arts.

This comprehensive and profound access to knowledge and education made Isidore of Seville one of the most influential scholars of the Middle Ages—and thus for Benno as well.

Isidore writes in Book XIX (on ships, buildings, and clothing), in Chapter VIII, that the instructores are the builders, while the architecti are the masons. He also mentions the maciones, who are scaffold builders important in construction. In Chapter IX, he explains that there are three phases in construction: dispositio (planning), constructio (erection), and venustas (adornment and decoration).

In Chapter X, Isidore elaborates on the process of building itself: the constructio of a building, the materials, the types of stones and their hardness, and which stone is suitable for which part of a building, among other details. He also describes how bricks are made and how to handle lime. Lime, according to Isidore, contains a hidden fire that is alive. One must ignite the lime with water (which normally extinguishes fire) and extinguish it again with oil (which usually fuels fire). In detailed descriptions, Isidore outlines the types of columns, which materials are suitable for foundations, and the volumes of water pipes can carry. While the structure may seem strange to modern readers, it follows a specific medieval logic. In other chapters, Isidore discusses decorative elements such as reliefs, mosaics, sculptures, and paintings. He also goes into detail about the colors that can be used in construction. Particularly interesting is Chapter XVIII, in which he explains the masonry tools. He describes the use of plumb lines and measuring cords, without which nothing straight can be built. Finally, in Chapter IX, he explains the woodworking trades and the connections between materials and construction. Not only does Isidore provide information on tools like compasses, but also on tools such as saws, axes, and drills, offering a highly practical overview of construction. Thus, he not only defines the meanings of terms for medieval Latin but also conveys accumulated, applicable architectural knowledge.

The third work from which Benno gained architectural knowledge is Vegetius‘ De Re Militari or Epitoma Rei Militaris.

Vegetius devotes his fourth book to the construction of fortifications. In the first chapter, he writes that cities and castles are fortified either by natural location or by art (manu), or ideally by both, and he goes on to discuss location. Castles and cities are especially protected when they are situated on a mountain or high hill, or surrounded by the sea, a river, or a swamp. Vegetius advises surrounding cities and castles with walls and ditches to protect them, and he describes in the second chapter how these should be constructed. A wall should not be built straight, to prevent it from being breached by siege machines. Instead, it should be curved and have angles where towers can be positioned, offering defensive advantages. In the third chapter, Vegetius explains how to construct such a wall. Two walls should be built 20 feet apart, with the space between filled with the excavated material from the ditches and compacted into a solid mass. The advantage, he continues, is that if the outer wall breaks, the compacted earth will act as a dam and prevent a breach.

In the fourth chapter, Vegetius addresses gates, which should be covered with leather and iron sheeting. A propugnaculum (a defensive work) in front of the gate would protect the structure. A portcullis could trap attacking enemies within the gate; openings in the walls above the gate could be used to pour water down to extinguish fires.

The fifth chapter is dedicated to moats, which should be wide and deep. This would make them difficult to overcome, for example by filling them in, and would also prevent undermining.

In Chapter Six, Vegetius writes that a supply of armor and shields should be kept in the fortification to protect the defenders, especially from arrow fire. Shelters made of wood, fabrics, and especially hammocks could be used to create protective structures to deflect arrows. Stone-filled baskets should be placed on the walls to be dumped on attackers trying to climb in with storm ladders. Vegetius emphasizes the importance of sufficient food supplies in Chapter Seven, and in Chapter Eight he focuses on defense supplies. A large supply of bitumen, sulfur, pitch, and oil should be kept on hand to set siege machines on fire. Iron, coal, and wood were necessary for the production and repair of weapons. River pebbles were particularly suited for use as projectiles, according to Vegetius, and these should also be stockpiled in large quantities. Boards and nails should also be available to make repairs or to strengthen defensive structures. In the following chapters, the author discusses various topics related to the storming or defense of fortifications, such as wicker shields, assault huts, or siege towers. The knowledge conveyed here offers a comprehensive overview of castle construction and the relationship between defensive elements, forces, and reserves, drawing on a rich Roman heritage.

What conclusions can be drawn from the observations presented here? First, the supposed innovation in high medieval castle construction was not an innovation at all, but a renaissance. The late antique knowledge that had been preserved and circulated in libraries was simply read by insightful individuals who not only recognized its importance but also had the opportunity to put the knowledge into practice. Castles were not the result of a development based solely on practical experience. They were the product of intellectual reception of existing knowledge: a complex process involving military considerations, symbolic communication, and contemporary aesthetics, which would have been unthinkable without the rich heritage of antiquity.

To conclude, I would like to present a beautiful observation. Norbert reports in Chapter 13 of the Vita Bennonis about the Iburg Monastery, whose construction is also attributed to the industrious Bishop of Osnabrück. He writes that there are many signs that the mountain on which the monastery stands was strongly fortified in ancient times and that there were excellent buildings there. Even in his day, the mountain was still surrounded by three walls, and subterranean masonry (subterraneis aedificiis) was discovered daily. This alone, says Norbert, is proof enough, but:

„But it can also be proven from written sources, which consistently affirm that among the numerous castles of this land, whose ruins we still see today, these three were the most important and powerful: Eresburg on the border between Saxony and Hesse, Sigeburg on the Ruhr, and our Iburg. No one will doubt that Iburg, due to its favorable location, held great significance. But when the great and renowned Emperor Charlemagne, in years of struggle and mighty military efforts, endeavored to wrest this land from the delusions of paganism and incorporate it into his empire, it is said that Widukind, the Saxon king, a man of—so it is told—almost superhuman strength of body and soul, fought many battles against the Franks from this city, of which he was then the lord.“

After the victory over the Saxons under Widukind’s leadership, Charlemagne decided to incorporate the Saxons into the empire. Therefore, he issued a decree that churches were to be built, and castles were to be torn down.

“It is certain that, at that time, alongside other castles that were widely demolished, our mountain was also turned into a wasteland.” Here, we have the earliest evidence of castellological research in German territory—almost 1000 years ago. Thank you very much!