## 3 Results  
### Detection and Substantiation of Damage Events: Wall Collapse at the Citadel of Aleppo WHS  
Inscribed on the UNESCO WHS List in 1986, the Ancient City of Aleppo includes a wealth of urban heritage assets such as 17 th-century madrasas, palaces, caravanserais, and hammams, as well as the 12 th-century Great Mosque and the 13 th-century Citadel (Table 1). The latter is probably the most iconic monument. The large medieval palace lies at the top of the prominent and steep elliptical hill (in archaeological terminology 'tell'; about 150 m by 300 m wide, 425 m above sea level) which is a very distinctive landscape feature of the WHS, as observed in the ASTER Global Digital Elevation Model (Figure 2a). The palace is fortified with a fence of turreted walls crowning the entire tell (Figure 2b).  
The Citadel, and more generally the whole WHS extending for \(\sim\)365 ha across the old town (Figure 2a), has witnessed the consequences of armed conflict, with several incidents having occurred since the city became the theatre of the 'Battle of Aleppo' in 2012-2016. Accounts of war damages are reported in various documents (for example, References [17; 18]). With specific regard to the Citadel, the major damage observed from space prior to 2015 was that to the roof formerly protecting the excavations of the Temple dedicated to the [PERSON] [1].  
The site-specific question to address in this case was whether Sentinel-2 could be used to identify damages in the Citadel during the crisis. We analysed 114 Sentinel-2 images covering the Citadel of Aleppo from 8 July 2015 to 14 December 2017, a temporal and spatial subset of which is represented in Figure 3. In the Sentinel-2 image acquired on 11 August 2016 (Figure 3b) we detected a new distinctive feature along the northern slope of the tell consisting in a brighter strip (that is, an increased surface reflectance) extending from the section of the Citadel fortification walls behind the Citadel Museum up to the tell toe.  
This feature was not visible on or before 4 August 2016 (Figure 3a) but it was permanent in the following Sentinel-2 images (Figure 3c-e), therefore it could be established that this was not an imaging artefact or an ephemeral feature. The feature extracted from the Sentinel-2 time series is indeed the mark of a wall collapse event that happened exactly on the date the Sentinel-2 image was acquired, that is, on 11 August 2016. As shown in a video published on YouTube on that date [28], the section of the Citadel fortification walls, once stretching between the Citadel Museum and the Great Mosque of the Citadel, collapsed on 11 August 2016. Further confirmation of the characteristics of the event was retrieved from the photographs and a video posted by APSA2011 on their website that were taken from outside the Citadel a few days after the collapse [29].  
Figure 2: (a) Boundaries of the designated UNESCO World Heritage Site of the Ancient City of Aleppo displayed onto a 30-m resolution ASTER Global Digital Elevation Model (GDEM), with an indication of the elliptical tell of the Citadel. (b) View of the Citadel walls as of 1 August 2005 (photo source: Wikimedia Commons). ASTER GDEM is a product of METI and NASA.  
Further corroboration that Sentinel-2 captured the collapse when it happened comes from the examination of a DigitalGlobe VHR image acquired on the same date and accessed through Google Earth (Table A1 in Appendix A). This image shows the same collapse feature and allows a more precise delineation of the footprint of the damaged area (red boundary in Figure 3g) to match the satellite-based assessment done by ASOR [30].  
However, it is not currently possible to update the condition of the collapsed section of the walls after 24 December 2016 via Google Earth. On the contrary, the systematic monitoring with Sentinel-2 enables us to observe the feature of the wall collapse persistently in the imagery until the end of 2017 (Figure 3f) and therefore, infer that the condition of the wall has not changed in the present day.