## 4 Discussion  
The demonstration on the 2016 wall collapse in the Citadel of Aleppo WHS is an example of systematic examination of the regularly acquired optical satellite imagery, even at HR, that can provide objective evidence to detect impacts of damaging events on the conservation of local heritage assets, and substantiate incident reports and in situ observations. It is, of course, understood that we were able to capture this damaging event through the systematic monitoring with Sentinel-2 against the baseline reference of the Citadel of Aleppo because the collapse feature was clearly visible to the satellite, and its size and extent were large enough to be resolved at a 10 m spatial resolution. It can be argued that, under different conditions, no feature could have been detected with Sentinel-2 due to its spatial resolution. In this regard, we recall the previous collapse that occurred along the eastern section of the Citadel walls on 11 July 2015. This event was reported and documented in the broadcast media the following day [32] and also by ASOR [33]. Based on published photographs taken from outside the Citadel, it can be observed that the collapse affected a few metres of the walls and the rubble accumulated in the upper part of the slope, just at the toe of the former walls. Therefore, this collapse generated a smaller event footprint than the event in August 2016. Google Earth does not provide an image taken on the day or soon after the 2015 collapse happened (see Table A1 in Appendix A). We can only compare two DigitalGlobe images acquired on 15 December 2014 and 26 October 2015 (Figure 9a,b) to analyse the collapse at a VHR. Although the visibility and illumination conditions are not ideal, the 26 October 2015 image somehow allows for the identification of the area affected by the collapse that occurred in July (Figure 9b). A concave feature is also visible at the toe of the collapsed wall section that was not visible previously. Figure 9: View of the eastern walls of the Citadel of Aleppo (**a**) several months before (15 December 2014) and (**b**) a few months after the 11 July 2015 collapse (26 October 2015). The red rectangle indicates the area affected by the collapse and the concave feature that developed on the slope after the collapse. (**c**) Grey-level representation of Sentinel-2 image acquired on 16 February 2016 with an indication of the area of higher surface reflectance probably caused by the weathering and erosion of the slope and rubble that moved further down the slope (green rectangle). (**d**) The VHR Google Earth image acquired on 20 March 2016 confirms the presence of the slope mark identified in the Sentinel-2 time series (yellow dashed rectangle). Google Earth images in (**a,b,d**) © 2018 DigitalGlobe; Copernicus Sentinel-2 image in (**c**) © 2016 European Space Agency—ESA. The comparison between the Sentinel-2 images acquired on 8 July 2015 and 17 August 2015 suggests that the collapse may not have been detected with Sentinel-2 without a priori knowledge. However, if the lower resolution of Sentinel-2 was a limiting factor for the event detection, subsequent Sentinel-2 images proved the benefit of having systematic acquisitions to follow the cascading effects of the wall collapse on the condition of the tell slope. By screening the Sentinel-2 dataset, it can be observed that the condition of the eastern slope of the tell does not change until 16 February 2016 when a feature resembling the track and toe deposit of collapsed debris becomes distinctively and permanently visible (Figure 9c). This suggests that, in the absence of remediation works and due to the exposure to weathering, the slope further eroded and some rubble moved further down the slope. DigitalGlobe images acquired on 20 March 2016 and 23 March 2016 validate our Sentinel-2-based assessment (Figure 9d), thus, implicitly providing proof that Sentinel-2 is valuable to track relevant changes in the heritage site beyond the detection of a single event.