Structure 10 (ST 10) was identified on the western half of the plateau, more or less on the SW angle of the plateau (cf. fig. 3). Its exposed situation on an outcropping rocky promontory, as well as its rectangular, almost square plan, might indicate that it was a watchtower. This would very well fit with its apparently carefully chosen position. The structure stands at a point with a perfect view all around the area, and especially towards all the springs supplying the long distance water supply of Petra: from ‘Aïn Tibitbi in the Baydha area to ’Aïn Braq on the road to Taybeh, including ’Aïn Mousa and the ancient settlement of el-Gji (Gaïa). The view from that spot is even more extensive, including the prominent hill of Dilagha at the very SE, itself occupied by a huge watchtower and giving access to the track leading to Gharandal, Jabal Haroun to the SW and the Wadi Araba to the W as well as Jabal Qaroun to the N. From some spots on the NW tip of Umm al-Biyara one can even see Qasr Umm Rattam, a major guard post controlling the access to the region around Petra from Wadi Araba On Umm Rattam and its strategic importance see Lindner et al. 2007; Lindner et al. 2000. . Although built directly on the visible rock surface, a certain amount of pottery was picked up on the surface in and around ST 10. With the exception of a very few Edomite sherds, the vast majority of pottery belongs to the Nabataean period, indicating a Nabataean date for ST 10. In general terms, the summit of Umm al-Biyara offers an excellent strategic view around the area, being in a sense the key to the functioning and existence of Petra in the Nabataean period. As pointed out elsewhere Schmid in preparation. , the city centre of Petra is actually built in a very illogical and uncomfortable spot. There is absolutely no view from the city centre to the surrounding areas, which was a major strategic disadvantage. Further, there is no regular natural water supply for a substantial population. In other words, a more logical place for developing a settlement would seem to be the area along the above mentioned springs, allowing the supply of a bigger population, offering a good view and being in the vicinity of possible agricultural areas. In order to settle permanently and to develop a substantial population in the city centre of Petra, immense efforts in constructing an effective infrastructure are necessary. Water management was surely one of the main concerns See for instance Bellwald 2008; Schmid 2008; Bienert 2002; Muheisen 2009. . Therefore, the Nabataeans invested an extraordinary amount of time, energy and finance in the creation of dams, channels, cisterns etc., both for protecting the city from flash floods and bringing fresh water into it. While these efforts in terms of water management mainly concern a kind of a half circle towards the E of the city centre, in terms of strategic concerns one has to control a much wider area in order to be able to live sustainably at Petra. Since from the city centre no view and therefore no communication with the wider area is possible, the Nabataeans needed to make sure they controlled the surrounding hills. By far the best view towards all important strategic points and the city centre is offered by the plateau of Umm al-Biyara, linking the city centre to the outer world. Not only is the view from Umm al-Biyara imposing, also the acoustic connection to the surrounding area is truly impressive, as can still be verified today. For example, from the summit of Umm al-Biyara donkeys can be heard from all over the city centre up to Umm Sayhoun, cars can be heard from Umm Sayhoun, Wadi Mousa, and the road to Taybeh as far as the hospital, and even people shouting within the city centre can be heard.
In conclusion, whoever wanted to dwell in the city centre of Petra on a permanent basis needed to be in control of Umm al-Biyara. This premise is important when it comes to interpreting the structures from the plateau belonging to the Nabataean period. That in general terms such reflexions as to strategic views and long distance communication played an important role for the Nabataeans is confirmed by Diodorus Siculus who, when reporting on the events of 312/11 BC, indicates that the Nabataeans had guard posts on elevated spots controlling the routes towards Arabia which communicated with each other using signal fires until the message reached „the rock“ (Petra) Diod. 19, 96, 3. 97, 1. . It is likely that this kind of strategic and spatial organisation was still in use some years later, though in a more developed way. This may be concluded from an episode dating to about 27 CE when the daughter of Aretas IV, who was married to Herod Antipas, fled to Petra, travelling from one Nabataean strategos to the next Jos., Ant. 18, 112). .
