Combining geophysical and geomorphological data to reconstruct the development of relief of a medieval castle site in the Spessart low mountain range, Germany

Within the Spessart low mountain range in central Germany, numerous castle ruins of the 13th century ce exist. Their construction and destruction were often determined by the struggle for political and economic supremacy in the region and for control over the Spessart's natural resources. Wahlmich Castle is located in a relatively uncommon strategic and geomorphological position, characterized by a fairly remote position and atypical rough relief. In order to reconstruct the local relief development and possible human impact, a multi‐method approach was applied combining two‐dimensional geoelectrical measurements, geomorphological mapping and stratigraphic‐sedimentological investigations. This provides new insights into the influence of landscape characteristics on choices of castle locations.

tively uncommon strategic and geomorphological position, characterized by a fairly remote position and atypical rough relief. In order to reconstruct the local relief development and possible human impact, a multi-method approach was applied combining two-dimensional geoelectrical measurements, geomorphological mapping and stratigraphic-sedimentological investigations. This provides new insights into the influence of landscape characteristics on choices of castle locations.
The combined geoelectrical, geomorphological and stratigraphic-sedimentological data show that the rough relief is of natural origin and influenced by regional faulting, which triggered sliding and slumping as well as weathering and dissection of the surface deposits. The rough relief and the lithology permitted intensive land use and building activities. However, the location of the castle offered access to and possibly control over important medieval traffic routes and also represented certain ownership claims in the Aschaff River valley.
The economic situation combined with rivalry between different elites led to the castle being built in a geomorphological challenging and strategically less valuable location. Focusing on castles located in rare and challenging geomorphological positions may therefore lead to a better understanding of castle siting in the future.

K E Y W O R D S
faulting, geoarchaeology, geomorphological mapping, geophysical prospection, percussion core probing, sedimentology

| INTRODUCTION
The location of medieval castles at prominent points in a larger landscape was often dictated by the requirements of military strategy and a desire to represent authority. In addition, the specific natural environment and its suitability in functional and structural terms, such as the distance to roads and settlements or the availability of water and security, determined the individual type and nature of a castle (cf., Kolb & Krenig, 1989;Ruf, 1984Ruf, , 2019Schäfer, 2000;Schecher, 1969). Within the Spessarta low mountain range in central Germany (Figure 1)there are numerous castle ruins of the 13th century CE (e.g., Gröber & Karlinger, 1916;Ruf, 2019). Several of these castles were constructed and destroyed during the territorial feud between the Counts of Rieneck and the Bishops of Mainz. The distribution of castles within the Spessart was generally related to population density, the economics of the medieval communities, and political decisions. All these factors are usually well known and discussed in historical and archaeological studies (cf., Kemethmüller, 2011;Rosmanitz, 2011;Ruf, 2019).
However, a comparison between the distribution of castle sites in the Spessart and the geomorphological setting reveals certain patterns as well. In the western Spessart, for example, a concentration of castles at a considerable distance from the Mesozoic Cuesta escarpment is evident ( Figure 2). Furthermore, there is a preference for castle sites along the Main River valley, while central and eastern Spessart harbours far fewer castle sites. Isolated inselbergs, however, are classic locations for military fortifications and therefore usually occupied by large castles.
To identify general locational factors and assess their significance, geoscientific evidence and historical and archaeological records of several castle sites should be compared. To examine the relationship of an individual medieval castle to its adjacent landscape, a more detailed spatial scale is required. The question arises as to the extent to which natural resources and ecosystem services determined castle siting.
In this context, Wahlmich Castle offers an ideal opportunity to gain new insights into the influence of landscape characteristics on the choice of a castle location in a clear and well-defined space by using a geoscientific multi-method approach. Situated in the Cuesta escarpment, Wahlmich Castle is located in a relatively unusual and almost unique strategic and geomorphological position, characterized by atypical rough relief. The objectives of this study are therefore (i) to investigate whether this particular relief is a natural feature of the Cuesta landscape or whether it was created by medieval building activity, and (ii) in doing so, to examine a possible influence of morphological characteristics on the choice of the castle location.
2 | STUDY SITE 2.1 | Regional setting The Spessart low mountain range is located in central Germany and can be divided along an articulated cuesta scarp into the Vorspessart in the west and the Hochspessart in the east (Figure 2). The largely uncovered bedrock of the Vorspessart comprises widely crystalline, Palaeozoic rocks mainly made of gneiss, quartzite and mica schist, whereas the crystalline rocks of the Hochspessart are mostly covered by Early Triassic rocks of Buntsandstein (or Bunter sandstone), which consists of sandstone with few conglomerates and claystone layers (e.g., Okrusch et al., 2011).
Wahlmich Castle is located in the Vorspessart, where the crystalline bedrock complex is made of Devonian Diorite and Granodiorite, classified according to its varying contents of orthoclase (Okrusch et al., 2011;Weinelt, 1962 (Käding, 1978). The bottom layer of the Bröckelschiefer, the so-called 'Basalbrekzie' (or basal-breccia; Okrusch et al., 2011), is defined as a mixed layer of Bröckelschiefer and the underlying material, in the case of this study the (Grano-) Diorite, which has no clear upper boundary (Scheinpflug, 1992). Tectonic activity is indicated by several faults, which strike predominantly northwest-southeast (NW-SE), following the general Hercynian trend that was active mainly in the Tertiary (Jung, 2006;Murawski, 1965;Weinelt, 1962). In addition, a north-northwest-south-southwest (NNE-SSW)-striking Rhenish trend exists that has been active since the Tertiary (Meschede, 2019) and defines the anticlinal structures of the crystalline basement of the Spessart (Okrusch et al., 2011).
Quaternary deposits of shallow depth occasionally cover the sedimentary and crystalline bedrock. These are mainly associated with Periglacial Cover Beds (PCBs), which developed during glacial periods of the Pleistocene and typically include structures like solifluction features, melt water channel accumulations and ice wedges (Meschede, 2019;Shishkina et al., 2019). The diagnostic properties of PCBs are specific structures of different movement processes, sorting characteristics and varying lithologic components and origins. Typically, three to four significant layers can be distinguished in the field (Ad-hoc Boden, 2005;Kleber, 1997;Kleber & Terhorst, 2013). PCBs of the crystalline Vorspessart area were studied and precisely described during the last years by Mueller (2011) and Mueller and Thiemeyer (2014 F I G U R E 2 Medieval castle distribution (a) and geomorphology of the Spessart low mountain range. Geomorphological regions represent simplified geomorphological regions derived from the detailed geomorphological classification provided by Jung (2006). Castle density (b) describes the number (n) of castles per 10 km 2 ; base map: hillshade from the ASTER DEM (ASTGTMv003 2019) [Color figure can be viewed at wileyonlinelibrary.com] Landau, 1958). Of particular importance were the rich stands of European beech (Fagus sylvatica) (Büdel et al., 2021;Lagies, 2005;Zerbe, 1997), which in combination with high water availability were an important prerequisite for highly profitable glass production (Ermischer, 2019;Wedepohl, 2003). Organized glass production thus commenced comparatively early in the Spessart region, in the 11th century CE (Wamser, 1979(Wamser, , 1982. Extensive mining activities and ore extraction have only been documented in the Spessart region since the 15th century CE (Freymann, 1991), however, there is some archaeological evidence of earlier iron ore extraction and processing (Hasenstein et al., 2019;Lorenz et al., 2010).

| Wahlmich Castle
The castle ruin 'Wahlmich' close to the village of Waldaschaff has been excavated and investigated since 2016. Wahlmich Castle was constructed around 1220 CE as a stone fortification with a few workshops and farm buildings on top of a small isolated hard rock hill surrounded by neck trenches in a hillside position . The castle was destroyed as early as the late 1260s

| METHODS
To investigate landscape development and human activity in the surroundings of Wahlmich Castle, a multi-method approach was applied, combining sedimentological and geomorphological studies with electrical resistivity tomography (ERT) measurements. In addition to core drillings, geographic information system (GIS)-based geomorphological mapping and terrain analyses were carried out that are commonly applied for site descriptions and the morphometric delineation and description of landforms (e.g., Siart et al., 2018). Based on the available geological and archaeological records of the study area Weinelt, 1962), the expected different lithological units should be detectable by differences in subsurface electrical resistivities using ERT measurments (e.g., Telford et al., 1990)

| Stratigraphic-sedimentological fieldwork
The stratigraphic-sedimentological fieldwork included the description of soils and sediments from soil prospection pits and coring sites.
Percussion core probing was conducted using a Wacker Neuson BH65 demolition hammer with drilling diameters from 30 to 80 mm.
Sediment description followed the German guidelines for soil description (Ad-hoc Boden, 2005). This included the identification of PCBs and the evaluation of their preservation, which depends on the intensity soil develoment and the occurrence and thickness of the specific layers (Mueller & Thiemeyer, 2014). Rock units were identified according to their characteristics as given in Okrusch et al. (2011).

| Geomorphological mapping
All mapping was performed in a GIS using standard tools of ArcGIS neighbourhood (Weiss, 2001). A multi-scale TPI image was generated with estimation window sizes of 25 m and 150 m in diameter using the SAGA GIS multi-scale TPI tool (Guisan et al., 1999).
The identification and description of landforms followed the modified rules of the detailed geomorphological mapping system for geomorphological maps of the Federal Republic of Germany at the scale of 1:25,000 (GMK 5; Leser & Stäblein, 1985). Subsequently, the landforms were affiliated to six levels, assigned as 'level of incision' (Champagnac et al., 2014;Shishkina et al., 2019). This expert-based classification delineates and classifies all land surfaces according to the form, slope and direction of their drainageways and enclosing relief levels and slope shoulders. The approach is based on the observation that slope erosion is directly triggered by accelerated incision into drainageways and gullies. Incision occurs not only in a vertical direction, but also in varying spatial patterns and densities. Both properties are directly related to the processes occurring on the accompanying hillslopes. Thus, the intensity of the slope processes can be quantified indirectly by means of the morphometric parameters of the mapped landforms. The level of incision therefore reflects the relief evolution of a defined area or set of landforms and semiquantitatively describes the rates of erosion and their spatial distribution. The higher the level of incision, the higher the rate of incision and erosion.

| Electrical resistivity tomography (ERT)
ERT measurements were conducted along five profiles in the surroundings of the castle keep in order to obtain information on the subsurface structure (Figure 5f). Measurements were carried out using a Syscal Junior Switch (IRIS Instrument S.A.S.; Orléans, France) and a Wenner-Schlumberger electrode array, due to a good signal-to-noise ratio (e.g., Dahlin & Zhou, 2004). All data points with a deviation of 5% or more between individual, reciprocal measurements were deleted in order to enhance data quality. ERT data were inverted To ensure the overall comparability of ERT depth sections, data from different profiles were displayed using the same legend categories for electrical resistivity, given in ohm-metre (Ωm) ( Figure 5). 4 | RESULTS

| Stratigraphical overview
In the rough terrain south of the castle keep, nine sediment cores were drilled to a maximum depth of 11 m below ground surface. In addition, two outcrops were investigated (for core and outcrop locations see Figure 3a).
As expected from the preliminary geological and pedological work in the area, different lithological units were observed and simplified to

| Geomorphological landforms, levels of incision and stratigraphy
The analysis of the DEM and the classification of the local geomor- described rough relief in the castle's surroundings, two low ridges can be recognized south of the keep (Figure 4a,b). East of these is another, larger ridge that appears rather dissected. It is not mapped as a single unit but is divided into three segments extending from the southeast slopes. In the following, all mapped landforms as well as type and thickness of observed stratigraphical layers will be spatially related to these levels (Figure 4b-d).
Incision level 0 is the 'baseline level' of the hillslope, which has not been significantly altered by erosion. It includes the gentle surface of the upper slope and the top of the castle site. The north-dipping hillslope south of the castle is extensively used and features several agricultural terraces of unknown age. At this level, the (Grano-)Diorite only starts at a depth of 10.7 m and the Bröckelschiefer has the greatest thickness. The latter is covered by rather thick soils in fully preserved PCBs (cf., core C1; Figures 3i and 4c,d).
Incision level 1 is covered by orchard meadows in the easternmost part of the study area, which are characterized by relatively gentle slopes and low surface roughness. Below the medium terrain step separating level 0 from level 1, the slopes already steepen and the surface rough-

| Electrical resistivity tomography (ERT)
In this study, five partly overlapping ERT depth sections are pres-  (Reynolds, 2011). One explanation for this discrepancy is probably the high degree of intensive chemical weathering and fracturing of the rocks. Furthermore, the great number of local springs within the study area indicates a high water content of the weathered (Grano-)Diorite. Both weathering and moisture changes can significantly reduce the resistivity of rock material (Reynolds, 2011).
However, the ERT measurements were not sensitive enough to detect the thin layers of the PCBs consistently in all profiles. This is probably due to the large electrode spacing (1.5-3 m) of the measurements (Reynolds, 2011). These were chosen in this study to reliably map the Bröckelschiefer-(Grano-)Diorite boundary assumed at about 10 m depth by previous investigations. The stratigraphic results of the cores and trenches therefore provide the more reliable data on PCB distribution in the study area.

| Spatial correlation of lithological units and landforms
The  Figure 6). Therefore, the relocation of the Bröckelschiefer, forming the relief south of the castle, seems spatially dependent on both the steep slope and a fault-system, which was partly mapped by Weinelt (1962).
The scarp of the fault is best visible in ERT profile E4 due to the occurrence of (Grano-)Diorite in the south. ERT profile E5 shows

| Landscape evolution
The particular spatial configuration of the crystalline Devonian Miocene as described in regional studies (Figure 8; Boldt, 1998;Jung, 2006). (1) The first scenario considers a theory proposed by Weinelt (1962 (Weinelt, 1962). The implied faulting activity then probably weakened the structure and texture of the bedrock, making it locally susceptible to intense Miocene weathering (cf., Jung, 2006;Okrusch et al., 2011).
(2) However, as evidence of sliding processes and intensive solifluction were observed in the study area, the occurrence of Several sunken roads exist near Wahlmich Castle that once connected the site and the Aschaff River valley with the important medieval traffic routes of several European trading cities (Himmelsbach, 2014;Landau, 1958). The individual roads thereby often follow the local land-use boundaries or lie far beyond the former farmland (Himmelsbach, 2005(Himmelsbach, , 2014 (Büttner, 1961;Franz, 2020). The claim to ownership of this area by the Counts of Rieneck could thus provide an explanation for the rather isolated location of Wahlmich Castle.
Overall, it seems that the economic situation combined with rivalry between different elites in the Aschaff River valley led to the castle being built in this unspectacular, almost remote and strategically less valuable mid-slope geomorphological position. This indicates that castle sites can also be expected in agriculturally unfavourable regions and/or at unique geomorphological sites, where other economic and political factors must have favoured the choice of location, even if they are difficult to prove historically.

| CONCLUSION
While anthropogenic influence on a landscape has often been discussed in science and proven in various case studies (e.g., Brown et al., 2017;Bork, 2020;Henselowsky et al., 2021), the example of the surrounding of Wahlmich Castle shows a different picture. Today, easily erodible ridges and valleys of weathered Bröckelschiefer form a rough landscape around the castle hill, which stands in strong contrast to the mostly gentle hills of the Spessart low mountain range. The analysis of sediment and geophysical data showed that regional faulting caused subsidence and subsequent fragmentation of the sediment body, which triggered the weathering and dissection of the surface deposits. The vertical offset caused by the faulting and the increased potential energy could also have triggered sliding and slumping of the Bröckelschiefer. Thus, despite the impression of a landscape that has been highly anthropogenically influenced and apart from building activity at the castle hill, the evidence supporting a natural origin of the relevant forming processes prevails. Accordingly, Wahlmich Castle is an anthropogenic structure built in a natural but exceptional geomorphological and geological setting.
In a supra-regional context, the location of the castle site close to conditions in such areas can be considered comparatively poor. It seems that focusing on geomorphologically rare and challenging locations may lead to a better understanding of castle siting in the future, and it may help to find more castle sites in comparable geomorphological and geological settings.
This study also demonstrates the importance of comprehensive geomorphological investigations for the identification and evaluation of castle sites in terms of factors influencing site selection. Furthermore, the thorough investigation of multiple interdependent landscape-forming processes also allows for a discussion of the constraints resulting from competing land-use pressures in particular landscapes.