View this artwork by appointment, at Tate Britain's Prints and Drawings Rooms
Cecilia Powell has identified the subject of this sketch as the town of Nemi from across the lake of the same name. Clearly visible are the circular tower of the Castello Ruspoli on the left and the arched substructures spanning the valley in the centre. The location of Turner’s view point was probably Genzano on the opposite side of the lake.
For a general discussion of Turner’s view of Lake Nemi see folio 19 (D15329).