{"nrhp_refnum":"70000540","name":"Fort Warren","other_names":[],"category":"district","location":{"address":"Georges Island, Boston Harbor","address_restricted":false,"city":"Boston","county":"Suffolk County","state":"MA","lat":42.31972127550716,"lon":-70.92807635470004,"geom_source":"nps_gis"},"listed_date":"1970-08-29","is_nhl":true,"nhl_designated_date":"1970-08-29","areas_of_significance":["Military","Architecture"],"significance_levels":["national"],"summary":{"what_it_is":"Fort Warren is a granite bastioned star fort on Georges Island in Boston Harbor, Massachusetts, begun in 1834 under Lt. Col. Sylvanus Thayer of the Corps of Engineers and completed during the Civil War. The 40-acre island is reachable only by boat.","why_significant":"The nomination calls Fort Warren one of the finest coastal fortifications built in the United States between 1816 and 1865 and the chief work of engineer Sylvanus Thayer. It is also regarded as the most important Civil War site in New England because it held Confederate prisoners, including James Mason, John Slidell and Alexander Stephens. The fort remains basically intact despite later modifications.","narrative":"Fort Warren occupies Georges Island, a 40-acre oval at the throat of Boston Harbor. Construction began in 1834 under Lt. Col. Sylvanus Thayer of the Corps of Engineers and was finished during the Civil War. It is a bastioned star fort built mostly of Quincy granite, with walls eight feet thick enclosing a parade ground of about four acres. Casemate guns, howitzers and Columbiads once armed its fronts. Later modernization in 1871-1876 and 1898-1899 added gun mounts, and a power plant and a World War I mine storage building also survive, yet the star fort remains structurally close to its mid-19th-century form. The fort is valued as a leading example of the Third System of coastal defense and as Thayer's chief engineering work. It is also considered New England's most significant Civil War site because Confederate commissioners Mason and Slidell, generals, and Vice President Alexander Stephens were held there. The state now manages it as a park.","period_of_significance":null,"architect_builder":"Lt. Col. Sylvanus Thayer, Corps of Engineers","architectural_style":"Bastioned star fort (Third System of coastal defense)","criteria":[],"status_note":"Rated in good condition and unaltered; administered by the Metropolitan District Commission, with a marina, picnicking and limited facilities, and open to visitors in summer.","vacant_flag":false,"machine_generated":true,"model":"claude-sonnet-5-5","prompt_version":"v1.0-2026-10-29","generated_at":"2026-10-29 02:11:53.859093-05","source_pdf":"https://s3.amazonaws.com/NARAprodstorage/opastorage/live/91/7938/63793891/content/electronic-records/rg-079/NPS_MA/70000540_NHL.pdf"},"photo":{"url_800":"https://nationalhistoricalregister.com/media/70000540/622660eee0-800.webp","url_1600":"https://nationalhistoricalregister.com/media/70000540/622660eee0-1600.webp","licence":"CC BY 2.0","author":"Doc Searls from Santa Barbara, USA","credit_html":"<a href=\"https://commons.wikimedia.org/wiki/File:Georges_Island_and_Fort_Warren_in_Boston_Harbor.jpg\" rel=\"noopener\">Doc Searls from Santa Barbara, USA</a>, <a href=\"https://creativecommons.org/licenses/by/2.0\" rel=\"license noopener\">CC BY 2.0</a>, via Wikimedia Commons","source_page":"https://commons.wikimedia.org/wiki/File:Georges_Island_and_Fort_Warren_in_Boston_Harbor.jpg"},"district":null,"sources":{"nps_snapshot_date":"2026-05-22","nara_catalog":"https://catalog.archives.gov/id/63793891","wikipedia":"https://en.wikipedia.org/wiki/Fort_Warren_(Massachusetts)","wikidata":"https://www.wikidata.org/wiki/Q2287837"},"indexable":true,"url":"https://nationalhistoricalregister.com/massachusetts/suffolk-county/fort-warren-70000540/"}