Most (28) of the WHH letters were written from Fort Thunderbolt (he called it Thunderbolt Battery), an earthenwork artillery fortification on the west side of a bend in the Wilmington River (then) about four miles southeast of Savannah. Today the site is private residential property in the suburban town of Thunderbolt (home of Savannah State University) with no recognizable features. Two detailed maps exist of Thunderbolt Battery and some sketches made for Harper's Magazine by William Charles Henry Wood.
This circa 1864 map shows Fort Pulaski and the town of Bonaventura where Fort Thunderbolt (not shown) was located.
This map from the National Archives shows Fort Thunderbolt in detail.
Front and rear of Fort Thunderbolt from sketchbook of William Waud (1832-1878).
Other sketches of Fort Thunderbolt.
Sketch of Fort Thunderbolt in Harper's Magazine, January 21, 1865.
Another sketch of Thunderbolt Battery
This 1864 map of Wassaw Sound shows Fort Pulaski but not Fort Thunderbolt.
Map illustrating the defence of Savannah, Ga. and the operations resulting in its capture by the army commanded by Maj. Genl. W. T. Sherman. Dec. 21st 1864 Compiled 1880-81 under the direction of Bvt. Brig. Genl. O. M. Poe, Maj. Corps of Engs., Col. & A.D.C., late Chief Engineer, Military Division of the Mississippi. Poe, O. M. (Orlando Metcalfe), 1832-1895. CREATED/PUBLISHED N.Y., Julius Bien & Co. photo lith., 1881
NOTES Scale ca. 1:85,000. Detailed map of Savannah and vicinity, showing "Union works in blue," "Confederate works in red," roads, railroads, vegetation, drainage, houses, names of a few residents in outlying areas, fences, and geographic coordinates. Includes plans of the following fortifications: Works at eastern point of Whitmarsh Island; Fort Thunderbolt; Bastion on the line of defence across Whitmarsh Island; Fort Tattnall; Battery on Turner's Rocks; Fort McAllister; Causten's Bluff; and Fort Lee.Map made for Gen. Sherman showing the defenses of Savannah.