A way to "guesstimate" the impact of removing all water can be, imo, attained by looking at : usgs.gov/special-topic/water-science-school/science/… and especially usgs.gov/media/images/all-earths-water-a-single-sphere : it shows how little all of earth's water really is (and how tiny liquid fresh water is)

You may want to compare its location to the historical "fertile crescent" : ancient.eu/Fertile_Crescent : it seems to be near (or even inside) it. I tried to find a specific view of it "2000 years ago" but couldn't find one yet, but compare the estimation circa 1350bc (ancient.eu/image/5435) to the current one : en.wikipedia.org/wiki/Fertile_Crescent, it is also kind of similar and not far from the fortress's location