## Materials and Methods  
To carry out our appraisal, we tested the Sentinel-2 HR imagery on the following heritage sites in Syria and Libya (Figure 1):  
* the UNESCO World Heritage Site (WHS) of the Ancient City of Aleppo in Syria (centre coordinates: 36\({}^{\circ}\)13\({}^{\prime}\)59\({}^{\prime\prime}\)N; 37\({}^{\circ}\)10\({}^{\prime}\)00\({}^{\prime\prime}\)E);
* the Hellenistic site of Apamea in Syria (35\({}^{\circ}\)25\({}^{\prime}\)11\({}^{\prime\prime}\)N; 36\({}^{\circ}\)24\({}^{\prime}\)05\({}^{\prime\prime}\)E), proposed for inscription on the UNESCO World Heritage List in 1999;
* the cultural landscape of Cyrene WHS and the modern town of Shahat in Libya (32\({}^{\circ}\)48\({}^{\prime}\)12\({}^{\prime\prime}\)N; 21\({}^{\circ}\)51\({}^{\prime}\)46\({}^{\prime\prime}\)E);
* the ancient Greek towns of Apollonia (modern Susah; 32\({}^{\circ}\)54\({}^{\prime}\)07\({}^{\prime\prime}\)N; 21\({}^{\circ}\)58\({}^{\prime}\)11\({}^{\prime\prime}\)E), Ptolemais (modern Ad Dirsiyah or Tolmeita; 32\({}^{\circ}\)42\({}^{\prime}\)24\({}^{\prime\prime}\)N; 20\({}^{\circ}\)57\({}^{\prime}\)10\({}^{\prime\prime}\)E), and Tocra (32\({}^{\circ}\)32\({}^{\prime}\)20\({}^{\prime\prime}\)N; 20\({}^{\circ}\)34\({}^{\prime}\)10\({}^{\prime\prime}\)E) in northern Cyrenaica, eastern Libya. Table 1 summarizes the heritage category, status and type of damage, site disturbance or land cover-land use change processes affecting the above demonstration sites. The background knowledge  
Figure 1: The geographic location of the heritage sites in the Mediterranean countries that represent the demonstration sites of this study (displayed on World Countries (Generalized), 2017 ESRI). about the processes under investigation is based on published and grey literature, outcomes of previous satellite-based assessments, and information extracted from the news, reports, photos, and videos found in the web and social media. Specific citations are reported throughout the text and summarized in Table 1. Our selection of demonstration sites aims to cover processes of potential threat to heritage conservation that occur in ordinary times as well as during times of crisis. Sentinel-2 data were analysed against different sizes, spatial extents, severities, and temporal evolutions of human activities, as detailed in Section 3. Our motivation to test the Sentinel-2 constellation for the condition assessment of heritage sites relies on the following image properties:  
1. Systematic global acquisition of multispectral imagery with consistent imaging parameters;
2. Spatial resolution of 10 m in the visible bands [12; 13];
3. Short revisiting time (up to 5 days) over Europe and the Mediterranean countries [24]. We used the Copernicus Open Access Hub to access the Sentinel-2 products acquired by the Multispectral Instrument (MSI) onboard the twin satellites Sentinel-2A and Sentinel-2B. This is the dedicated platform where Top-Of-Atmosphere (TOA) reflectance in cartographic geometry (that is, Level 1C, or L1C) products are regularly published by the European Space Agency (ESA) very soon after acquisition. Since April 2017, the Bottom Of Atmosphere (BOA) reflectance in cartographic geometry (that is, Level 2A, or L2A) products are also published 48-60 h after the L1C products, based on automated processing of the latter with the Sen2 Cor processor and PlanetDEM Digital Elevation Model [25]. For each demonstration site, the temporal period of our observation spanned from the first image available in 2015 up to December 2017. Of the 13 MSI spectral bands, we focused on the B2 (490 nm), B3 (560 nm), B4 (665 nm) bands and their Red-Green-Blue (RGB) colour composites at full resolution. We intentionally concentrated on these bands because the majority of archaeologists and practitioners assessing the condition of heritage sites based on optical satellite imagery mostly rely on the use of RGB information of VHR satellite images (for example, References [1; 3; 5; 6; 19]).