# Exploring Ephemeral Features with Ground-Penetrating Radar: An Approach to Roman Military Camps
## 2 Materials and Methods
### Case Studies
#### 2.2.3 The Siege of Cerro de Castarreno (Sasamon and Villasandino, Burgos, Spain)  
Since 2017, the Warscapes project has been investigating several Roman camps around the oppidum of Cerro de Castarreno [41] (Figure 4), which controls a gently undulating plateau (ca. 795 masl) close to the confluence of the Brulles and Odra rivers. The first archaeological features had been documented thanks to previous aerial [42; 43] and field surveys [44], although the discovery of the sites analysed here took place during the 2019 and 2020 campaigns. The archaeological features lay on a Holocene zone composted by gravel or loose rock, sand, and clay, influenced by the Brulles River (Figure 6). Areas farther from the river valley are of Miocene origin, similar to the geology of the Sueros de Cepeda camp. Figure 4: Layout of the analysed sites drawn on aerial photographs. **(1a)**. Alto da Raia (SPLAL 1949); **(1b)**. Alto da Raia (Google Earth 15 July 2020). **(2a)**. Sueros de Cepeda (USAF 1956); **(2b)**. Sueros de Cepeda (Google Earth 13 September 2017); **(3a)** Villa Maria (Sasamón) (USAF 1956); **(3b)**. Villa Maria (Sasamón) (Google Earth 17 June 2019); **(4a)**. Los Andinales (Villasandino) (USAF 1956); **(4b)**. Los Andinales (Villasandino) (Google Earth 17 June 2019). Note that the images are reproduced at different scales. Black arrows point to the _contravallatio_ system; white arrows point to the _circumvallation_. EPSG: 25830. Figure 5: The siege of Cerro de Castarreño. Late Iron Age _oppidum_ (blue triangles), Roman camps (red triangles), siege structures (black lines), and the extent of the modern-day town of Olmillos de Sasamón (white area). The areas analysed in this paper have been marked with red-dotted lines. EPSG: 25830. JMCG. The area is almost exclusively devoted to cereal agriculture (wheat and barley), although some industrial crops for vegetable oil cultivation started in the last few years. The inspection of the USAF imagery (1956) shows a pattern where small field plots were merged into larger units, thus impacting the preservation of archaeological features. Nevertheless, some of the siege structures were already visible close to the Arroyo del Puerto stream in 1956, soon before the land reorganisation started. Beyond a characterisation based on morphological parallels, these structures' exact functionality and dating were debated. The systematic aerial survey of the zone using UAVs and new satellite coverages released in 2020 fostered the detection of several traces  
Figure 6: Main lithologic units in the study areas (after IGN-MAGNA 50.000, sheets 160 and 199) and geological map of Portugal 1:50,000 from LNEG (National Energy and Geology Laboratory) (sheet 02-C–Tourem, 1977). The layout of the analysed sites has been drawn in every case. From left to right: Alto da Raia camp (ADR), Sueros de Cepeda camp (SDC), the siege structures around Cerro de Castarreno (VM-Villa Maria, and LA-Los Andinales). Images are reproduced at different scales. EPSG: 25830. Figure 7: Indication of GPR survey area in red over drone-DSM ([PERSON], University of Porto). Excavation trenches are marked in black. Depth slices from Alto da Raia’s Roman camp. (**a**) inner rampart; (**b**) berm; (**c**) _clavicula_; (**d**) external ditch. EPSG: 25830.  
related to the Roman military presence on a regional scale [7; 45; 46]. As a result, the evidence previously gathered in the Sasamon and Villasandino municipalities was reinterpreted as a possible siege scenario around the Late Iron Age _oppidum_ of Cerro de Castarreno with remarkable similarities to Alesia [47; 48]. A double ditch ca. 6 km long conformed the siege lines facing the hillfort--_circumvallatio_--while a similar structure--_contravallatio_--protected the rear-guard of the assaulting Roman army. The two defensive lines are frequently spaced between 55 and 100 m apart, while the distance between ditches within each defensive line is usually about 4 m. The troops were quartered in at least six military camps, of which two will be studied here.