Statistics for occurrence #1 of “Fort Warren” in chapter 1.2 of The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 7: Prisons and Hospitals.:

...uch accommodations were entirely inadequate. The capacity of the forts along the seaboard was limited, with the exception of Fort Delaware, and besides they were soon full of political prisoners. Fort Warren , in Boston Harbor, sheltered a number of Confederate privates during the first year of the war, but later was used chiefly for the confinement of political prisoners and general officers. Likewise,...
Max. Freq. Min. Freq.
Entity Corpus Doc Corpus Doc  
Fort Warren (Massachusetts, United States) 1,093 40 11 0 0 user votes

† This entity has been selected by the automated classifier as the most likely match in this context. It may or may not be the correct match.