Fort Warren
Why it's listed
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.
Significance in brief: 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.
Machine-generated summary. This summary was generated by a language model from the official nomination document and has not been reviewed line by line by an editor. It may contain errors. Check the source document before relying on any detail. Source: nomination document (PDF) · National Archives catalog entry. NRHP reference 70000540, listed August 29, 1970. Summary generated October 2026 (model claude-sonnet-5-5, prompt v1.0-2026-10-29, confidence medium).
Key facts
| Address | Georges Island, Boston Harbor, Boston |
|---|---|
| County | Suffolk County |
| Listed | August 29, 1970 |
| Reference number | 70000540 |
| Resource type | District |
| Architect / builder | Lt. Col. Sylvanus Thayer, Corps of Engineers |
| Architectural style | Bastioned star fort (Third System of coastal defense) |
| Areas of significance | Military, Architecture |
| Level of significance | National |
| National Historic Landmark | Yes, designated August 29, 1970 |
| Status (as stated in the nomination) | 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. |
| Acreage | 4 |
| Coordinates | 42.31972, -70.92808 (NPS GIS) |
What listing means for this property
This is a historic district. Buildings within its boundary are either contributing (they add to the district's significance) or non-contributing. Owners of contributing income-producing buildings can claim the 20% federal rehabilitation tax credit; homeowners should look at state credits. National Register districts impose no design review by themselves; a local historic district ordinance, if one exists, does (the difference explained).
General information, not legal or tax advice. Rules change; confirm with your State Historic Preservation Office or a qualified adviser.
Sources and further reading
- National Park Service, National Register of Historic Places database (snapshot May 2026), reference 70000540.
- Nomination form (PDF, National Archives) — catalog entry
- Wikipedia article (used for cross-checking only; text not reproduced here)
- Wikidata Q2287837
- This listing as JSON
Last updated October 2026. Found an error? See the editorial policy for how to report it.