An very interesting piece by Dr Dorel Bondoc, expert on Archaeology from Oltenia Museum, Craiova, Romania, about the history and archaeology of the submerged island in the middle of the Danube. It refutes Paddy’s claim in BTTW that the mosque was moved.
by Dr Dorel Bondoc
Published on the Alexis Project website.
At the beginning of the 15th century, the island was occupied by Turks, who understood its remarkable strategical importance for the development of the river trade on the Danube, after the exit from Kazan region. In 1718, as a result of the treaty from Passarowitz (Pojarevat), the northern Serbia, Banat and Oltenia became possessions of the Austrians, as was also the case of Ada-Kaleh island which then bore the name New Orsova.
The Austrians built a strong fortification of “Vauban” type on the island. In 1739, after the treaty from Belgrad, Austria returned Serbia and Oltenia to Turkey. As a result the island was occupied again by Turks, who gave it the name Ada-Kaleh, which may be translated as “the island of the fortress.” The toponym can also be found in the documents of that time as: Ada Kale, Ada Cale, Adakaleh, Ada Kaleh or Adacale.
Read more here.
Patrick Leigh Fermor 
I’m guessing that as a hydro-electric project , there’d have been a lot of demolition on the island prior to flooding it , simply to get rid of all the trees and construction timber in the buildings , because you dont want great hunks of roofs , ceilings , floors , balconies , or tree trunks , to come free and jam as waterlogged detritus in the intakes to the turbines , or water level control sluices , or shipping Locks , and so on .
I remember seeing a film of the Kariba Dam construction during the late 1950s where caterpillar tracked bulldozers were dragging vast rolling balls connected by huge chains through what would become the Kariba Lake bottom to chew all the trees down and clear the scrub away .
The bits in the linked articles about there having been a Roman fortress there is interesting , of course many military sites have continous occupation , with one layer of new fortifications going on top , or into , what was it’s predecessor ,as the strategic nature of a site outlasts the previous era’s technology .
I suppose a good local example would be Dover Castle , where the concrete pouring only stopped in the 1960s – over and into defenses dating from WW2 , WW1 , Victorian , Napoleonic , Tudor , medieval , Norman , Saxon and Roman periods . There is probably an Iron Age Hill fort under that lot too .
Very good archive collection of images of Ada Kale in the articles . I suppose today it’d have been declared a UNESCO World Heritage site , pity it’s gone .
I have wondered how deep is the water over this area?
I wonder if diving expeditions are available to explore the ghost town maze-like streets? Or is it too treacherous with currents and water traffic? It would be an eerie experience.