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How to build a medieval fortress – the construction of Hildagsburg

It’s hard work building a fort…just ask the Ottonians…

This Early Medieval dynasty of German kings ruled during the 10th and early 11th century and built a lot of fortresses! In a six-year period between 929-936, Henry I “The Fowler”, built three dozen fortresses! But why?

Fortresses were built as part of a well planned defensive system. In an effort to maintain control over the German kingdom, and to protect their frontiers from Slav incursions, the Ottonian kings built many fortifications. Unlike Cathedrals that could take years or decades to complete, these fortresses had to be built quickly because they were needed for protection or military campaigns. They were massive and expensive undertakings, which required enormous resources in order to construct hundreds of fortresses. The decision to build such fortifications lay with the Ottonian kings because it had serious political, military and economic consequences. So what exactly did it take to build a fortress?

Moats

Hildagsburg was a double fortress; it had a walled structure (Hauptburg, in German), and a large earthen berm (a raised bank, or protective ridge), and 3 moats (2 of which were integral to the fortress’s defence), along with an outer bailey, called a Vorburg. Ottonian fortresses employed complex defences for protection. Their fortifications were equipped with moat-like ditches, called fosses, that were partly filled by water like those of their Roman, Merovingian and Carolingian predecessors.

Hildagsburg required of 30,000 cubic metres of earth, stone and vegetation to be removed in order to excavate the ditches for its 3 moats. How big were they? The inner moat was 450m long, 4m wide, 2m deep. The outer moat was slightly larger at 470m long, 4m wide, 2m deep. The 3rd moat was massive, measuring 800m in length, 11m in width, and 2m deep. Engineers created a continuous hydrological system to feed the moats with water.

Stone Walls

The stone walls of of the Hauptburg were constructed of a material called Greywacke, Grauwacke – a hard sandstone from the Harz mountains. The stone was cut into blocks that were similar in size to modern day bricks. There were over 500,000 greywacke bricks used to build the Hauptburg at Hildagsburg and 262,000kg of sand had to be dug up from a river bank to make the mortar!

Manual Labour

The building of a complex wood and stone structure such as Hildagsburg required 2-3 million man hours of labour. The experts behind these fortifications had to have some understanding of basic military defensive structures, how to design everything according to the king’s wishes and, how to make the correct estimations for the skilled and unskilled labour required for the task.

If the fortress was in a dangerous area, the planners also had to correctly estimate the number of troops that would need to be deployed to guard the work site. At approximately 2-3 million man hours, 2,700 men, working for one hundred 10 hour days, would take approximately 4 months to finish the entire fortress. This doesn’t account for inclement weather or other problems that may arise, but is a best case scenario. The manual labour required for the moats took approximately three thousand 10 hour work days for the diggers, and an additional three thousand 10 hour work days for their helpers.

Technology

There is a popular misconception that there was little to no knowledge preserved from the Roman period to the Early Middle Ages, giving rise to the erroneous title, “The Dark Ages”. This is far from true, and early medieval fortifications are a great example of the continued connection between the two periods. There is evidence that the Ottonians used Roman as well as Early Medieval manuals and surveying handbooks when building their fortifications. The sophistication and level of difficulty in the triple moat system as as well as the their complex hydrology points the the fact that there was a definite continuity of knowledge and building techniques in the Early Middle Ages from Roman Times.

Wooden Walls

The wooden walls consisted of overlapping logs. A good example of a similar type of fortress would be the Early Medieval stronghold of Raddusch. For its wooden walls, 1,221-1,628 oaks were used. The wall was 144m in length and 11m wide. Hildagsburg, being a double fortress, meant that its walls were 800% greater, requiring 8 times the number of trees, that’s 11,000 oaks! How long would this take to construct? Modern day woodcutters can fell 3 oaks trees per day. 11,000 oak trees would take 3,700 days under modern conditions. During the Middle Ages, with poorer axes, it would have taken 7,400 days. It’s most likely that they use trees in the immediate vicinity to avoid the extra time involved in transporting the logs to the building site.

You can read more about this topic in the article “The Costs of Fortress Construction in Tenth-Century Germany: The Case of Hildagsburg,” by Bernard Bachrach and David Bachrach, which was published in Viator, Vol.45:3 (2014).

Both Bernard and David Bachrach are leading historians of medieval warfare We interviewed this father-son duo in 2010:

Top Image: 16th century stonemason at work

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