Palace Research in Aachen

  Copyright: © L. Hugot 1965 Fig. 1 Reconstruction of the Aachen palace  

Department of History of Architecture and Conservation
Department of Historic Building Conservation
Dr.-Ing. Judith Ley, Dipl.-Ing. Marc Wietheger


Aachen was the most important palace in the Carolingian Empire. From the second half of the 8th century onwards, the royal court more frequently spent the winters there, celebrated the big festive days and received envoys from all over the world. According to its status as a “sedes”, i.e. the seat of the king and his court, the imperial palace of Charlemagne and his son Louis the Pious was endued with representative buildings. Being the coronation site of the German kings, it eventually became one of the most important centers of power of the entire Middle Ages.

Apart from the Palatine Chapel, now Aachen Cathedral, there are other remains of the stone-built palace buildings (Fig. 1). The Aachen Town Hall, for example, stands on the foundations of Charlemagne’s Council Hall (Fig. 2). On its eastern side rose the so-called Granus Tower. Assembly hall and church were connected by a two-story corridor, which was divided by a transept. In addition to the atrium in the west, there were further annex buildings to the north and the south of the church. It is not clear which functions Granus Tower, transept and annex buildings had in the structure of the palace complex. On the other hand, sources report on numerous additional buildings whose location is unknown.
In spite of the apparent importance of Aachen in the early Middle Ages, today, one can only speculate as to the appearance of the palace, the functional relationships within the palatine district and the architectural concept of the complex. This topic was last researched during the 1960ies. The last major reconstruction of the Aachen palace by Leo Hugot originates from this period. It is widely unknown, however, that this reconstruction only reflects a preliminary result of the research at that time.
Today, the fact that documentation of the original basic building fabric is still lacking and that the processing of the results from earlier excavations and the traditional written sources is rather inadequate, leaves much to be desired. There is neither any systematic analysis and classification of the Carolingian designs nor are there any theories about how the importance of the palace was updated in the architecture of the following eras.

  Copyright: © M. Merian 1647 Fig. 2 North facade of the Aachen city hall  

An extensive research cooperation of several departments of RWTH Aachen University, the City of Aachen Office for Historical Monuments and Urban Archaeology, is supposed to close these research gaps in the course of the next few years.
A detailed documentation of the existing profane palatine buildings and the review of the corresponding archival records is carried out in a cooperation between the Department of Historic Building Conservation and the Department of History of Architecture. Funds from the “Investment Program National UNESCO World Heritage Sites: Palatine Research from the Perspective of Building Research” aided by the federal government and City of Aachen have been made available for this purpose (Prof. Dr.-Ing. Christian Raabe, Dipl.-Ing. Marc Wietheger). Also, the German Research Foundation (DFG) promotes the project “The Aula Regia in Aachen – Carolingian Council Hall and Late Medieval Town Hall” (Dr.-Ing. Judith Ley), which aims to analyze and classify the medieval building substance within its historic building context. Furthermore, the research works are significantly supported by the Town Hall Society.

The Department of Medieval History is responsible for the reviewing of the former excavations and the written sources . Funds will also be made available by the „Investment Program National UNESCO – World Heritage Sites” for the archaeological investigations (Prof. Dr. Harald Müller / Privatdozent Dr. Sebastian Ristow). In the light of these new research results, the written sources will be analyzed in the as yet missing volume about Aachen in the “Repertory of the German Royal Palaces”. As part of these projects, interdisciplinary seminars will be offered by the mentioned departments, which not only enable students to gain in-depth insights into modern research methods, but also to get to know the benefits of interdisciplinary and practice-oriented networking.
This interdisciplinary knowledge transfer will be coordinated by the study group Palatine Research at RWTH. It joins up all institutions which work within the Aachen palatine context. In addition to the above mentioned departments, these are the Administrative Office of Conservation, the Urban Archaeology, the ICOMOS representatives from the Department of History of Urbanization, the Town Hall Society, the Rhineland Regional Authority, the Cathedral Works and members of Board of Trustees of the Charlemagne Exhibition 2014. An outcome of this collaboration will be a computer model of the palace, which for the first time classifies the conserved buildings chronologically and places the archaeological results. In addition to the modern building and excavation documentation, results from previous excavations, archival materials and estates are integrated into this model. Only then is it possible to provide an overview of the connections in the palace to enable a new reconstruction attempt of the entire site.



National Investment Program UNESCO- World Heritage sites:
Palatine Research from the Perspective of the Building Research
Department of Historic Building Conservation
Prof. Dr.-Ing. Christian Raabe, Dipl.-Ing. Marc Wietheger,
Dipl.-Ing. Robert Mehl


The described research project began in 2007 as a student seminar on building documentation and building research of the Granus Tower (Fig. 3). Since, the building has been systematically measured, documented graphically as well as photographically and described scientifically by architecture students led by Marc Wietheger and Judith Ley in several one-week campaigns.
By means of tachymetric, CAD-based online measurement of the architectural geometry, photogrammetric recordings of stonefaced surfaces and manual measurements of detail areas digital true-to-deformation plans were created. By this means all the main and mezzanine floors were recorded within a consistent measurement reference system (3D-fixed point grid) in detailed floor plans as well as vertical sections on a scale of 1:1 (Fig. 4).

The plans show that the outwardly unimpressive looking tower shows a complex spatial structure in its interior: The lower four floors from the Carolingian period reaching up to a height of about 20 m, feature barrelvaulted stairways enclosing square interior rooms which are spanned by cloister vaults (Fig. stair and interior Granus). However, this is neither carried out in a regular continuous spiral, nor by floors of the same height. Rather, the passage in the first and third floors was directed alongside high ceilinged rooms, on the second floor past a low ceilinged room. On this floor, the flights of stairs diverge, so that a blind alley had to be created as a rest and relief space between them.

In medieval sources, this unusual tower is referred to as “turris regia” or “saaltorn”, which emphasizes its relation to Aula Regia. The original function of the tower, however, is unknown. Because of this unusual structure and the existing interior rooms, presumptions range from defense tower to residential tower of Charlemagne to treasury.

The most recent surveys show that the greatest effort was invested in the construction of the staircases. The flights of stairs were lit by way of several windows so that they could be used without any additional artificial light during the day while the interior rooms were only sparsely lit by indirect light. Building the staircases wide around these rooms created a rise which was comfortable for medieval standards. In addition, the comfortable width of the treads, the representative soffit of the tower entrance door and several decorative columns show that the staircase must have had an important function within the structure of the Palace.
Thus we are faced with one of the first representative staircases north of the Alps – even though, because of the conservation status of the auditorium, we ultimately cannot say where the stairs actually led.
Building on these basic preliminary investigations, the funding of the current research project was successfully applied to at the Conservation Office of the City of Aachen in 2010, so that the advanced work at the Granus Tower could be intensified and extended to the historic Town Hall (Fig. 7). In addition to the complete and accurate architecture measuring, the building description, as well as the necessary scientific restorative investigations, all archives are closely inspected.
In order to ensure the direct comparability of the new documentation results for the entire Palace, these surveys are guided by the research performed on the Palatine Church by the regional authority Rhineland and the Aachen Cathedral Works in accompaniment to the restoration work carried out between 2000 and 2006 – especially in method and indexing of material mappings and samplings.


In 2011, the exterior facades of the Granus Tower were thoroughly investigated in due consideration of these requirements. In order to enable this research task, the building management of the City of Aachen at the beginning of the year authorized that the scaffold erected on all four tower facades for the rehabilitation of the pyramidal tower roof was equipped with additional levels. At the same time, the estate of Leo Hugot, who had begun to explore the Town Hall in the 60s and 70s, was handed over to the city by his family. It contained unpublished stone-by-stone measurement plans of different sections of the tower’s exterior walls. These not only allowed a first uncomplicated material and building phase mapping but also showed the tower walls in the state before their extensive renovation after World War II. To document the current condition of the walls, high resolution measuring images of the tower facades were taken as the basis for the final plans (Fig. 6).
For the description of the geometry and materiality of the floors, walls and vaults in the Granus Tower, a 3D scanner was used, which helped to clearly illustrate the complex relations within the narrow parts of the tower’s interior (Fig. 5).
The results of the systematic re-documentation as well as the numerous refurbished archive documents will be brought together in a digital data model in the course of the project, which will also include the archaeological findings of the palatine district. This 3D building-information system (GebIS) which is based on exact plans and visually associated with space related data, will facilitate the cross disciplinary management, analysis and supply of all construction information even beyond the project grant programme. In addition, part of the information should be made publicly accessible through a web portal.

Another essential result of the works on the entire Town Hall will be a timetable of construction phases, specifying which construction methods with which effects have been conducted at what time in the entire complex of the Town Hall. Thus, the genesis of the building as it is today will be understandable as a whole for the first time. Finally, we will arrive at a genealogy and building historical classification of the Town Hall in three volumes, which correspond to the three main eras of these changes: Middle Ages (Judith Ley), baroque (Georg Helg) and 19th and 20th century (Marc Wietheger).



The Aula Regia at Aachen: Carolingian Assembly Hall
and Late Medieval Town Hall
– Historical Building Research and History of Architecture
Department of History of Architecture and Conservation
Dr.-Ing. Judith Ley

Copyright: © M. Wietheger Fig. 7 Ground plan of the coronation hall

With Charlemagne’s Assembly Hall, Aachen Town Hall holds within its basic fabric one of the most important medieval stately buildings of the German-speaking countries. In the tradition of this King’s Hall, the building was profoundly restored several times for the Aachen coronation ceremonies of the German kings in the Middle Ages and modified according to the representational requirements of the particular time.
The power-political rank of Aachen was always high, which raises the expectation that the development of the Aachen Town Hall from the Carolingian King’s Hall reflects very clearly how the construction type of the stately hall was modified from the ancient world until the end of Middle Ages. As of today and despite its importance, there has not been any systematic investigation and analysis of its building fabric nor has there been a critical interpretation of its repeatedly changed stylistic vocabulary.
The aim of the research projects funded by DFG since January 2011 is a coherent building historical exploration of the medieval building and its changes. Based on the modern construction documentation described above, the first task is to offer well-founded reconstruction suggestions for the different medieval building conditions. The question of which position the Assembly Hall at Aachen takes in the development of stately architecture in Europe, will finally be addressed by a juxtaposition with other ancient and medieval halls and palaces. The conception of ceremonial spaces and the iconographic and iconological interpretation of the stylistic vocabulary are as much to be examined as the derivation of the applied and often complex building constructive solutions.

The Carolingian original building from the end of the 8th century, not only determined the tradition of the place but also the dimensions of the building (Fig. 1). With this design, the Aachen Assembly Hall stands in the tradition of ancient government halls of profane and ecclesiastical dignitaries, which during the 8th century were still in use or even newly built in Rome and Byzantium but also in many other places.

For the reconstruction of the rising architecture of the assembly hall, there are only a few clues, because the basic building fabric of the hall has been largely removed so that the Granus Tower is its highest preserved part. Thus, the question whether the hall was divided into several levels could not be answered conclusively. It is an important question insofar as the Carolingians consolidated the principle of attaching superior significance to higher floor levels, which even today is generally understandable throughout European architectural language. Therefore, the question is, whether the King’s Hall had a second level - possibly in correspondence to the gallery of the Palace Chapel. A hint to such a scaling of heights is given by the representative staircase in the Granus Tower.

  Copyright: © C. Scheuren 1877 Fig. 8 Coronation hall with the planned emperor figures  

Like the Palace Chapel, the Granus Tower shows that when the palace in Aachen was built, the idea of the Roman monumental building was combined with the smallscale cell-type building technique of the Germanic architectural tradition. This means that now, unlike during ancient times, the parts of the building no longer melted into one large room, but single room sections were clearly distinguished from each other in both the horizontal and the vertical division of the building. Also, they often had very different functions. Thus, the Granus Tower is not built like an Italian Campanile with an evenly, “infinitely” ascending staircase around a central shaft, but divided into individual tower sections by means of lockable interior rooms, which were of different heights and spanned by cloister vaults, as well as by changes in the rotational direction of the staircases (Fig. 4). In a manner of speaking, it consists of several staircases, which were piled onto each other to connect the different floors of the adjacent building elements respectively to close them off from each other like a sluice.


Like the Palatine Church, the King’s Hall probably experienced its first redesign already during the Romanic period, when Aachen proceeded to become the coronation site of the German kings. So far we cannot tell, however, how profound this reconstruction really was and which role it played within the Romanesque palace buildings, as found for example in Goslar or Braunschweig, which were always constructed with two stories.
Even when the city took over the meanwhile dilapidated building in the 14th Century and rebuilt it into the Town Hall, the older palace function was inherited by the integration of an impressive festival hall for the coronation ceremonies on the first floor and the construction of a magnificent façade embellished with sculpture (Fig. 2, 7, 8). The combination of these two functions is the reason for the prominent role of the Gothic building, whose design is equally geared to German and French palatine buildings, inter alia the King’s Palace on the Île de la Cité in Paris (Fig. 9).

Copyright: © Reconstruction A. Huyskens at 1925 / E: Viollet-le-Duc 1864 Fig. 9 Comparison of the Gothic plant with the Palais de Cité in Paris

The huge Coronation Hall, which was spanned by stone vaults and should also have been embellished with sculpture, was adopted from the palace construction tradition and is therefore especially unusual for a town hall (Abb. 8). A particular technical challenge was to construct this wide spanning vault without disturbing the delicate façade by supporting pillars. Among other things, the transition to the Granus Tower had to be bricked up in order to achieve this. Thus, it lost its original function and finally comprised the apartment for the city guard, the city’s archive of documents and a prison.
Still, even when - from 1562 onwards - the German kings were no longer coronated at Aachen but only in Frankfurt, people were aware of the building’s original function. In 1647, Merian still super scribed his description of the Aachen Town Hall as “The Palatine and Town Hall at Aachen” (Fig. 2).