Outer Harbor: South Boston Beaches and Castle Island
A three-mile arc from Fort Independence's granite star to Carson Beach — Pleasure Bay's causeway, Sullivan's landmark hot dogs, and constant views of the main ship channel.
The Abandoned Stronghold
The low drumlin now known as Castle Island guarded the only deep-water channel into the inner harbor — a natural chokepoint where all shipping had to pass within musket range of the fort. By 1775 the works standing there were called Castle William: a masonry star fort mounting more than 100 guns, garrisoned by British regulars, and used as the secure refuge to which the customs commissioners and, later, the surviving soldiers of the Boston Massacre were withdrawn when the town grew ungovernable. Throughout the Siege of Boston it was the outer anchor of the British position in the harbor.
At a Glance
- Route
- Castle Island (Fort Independence) → Pleasure Bay → M Street Beach → Carson Beach
- Distance
- 3.0 miles
- Duration
- 60–75 minutes
- Difficulty
- Easy · flat, wind-exposed
Getting Here
- T Access
- Broadway (Red Line) + Route 9 or 11 bus
Amenities
- Rest Stops
- Sullivan's at Castle Island (Mar–Oct); Carson Beach bathhouse
- Admission
- Free · DCR-managed · seasonal ranger tours of Fort Independence
The Journey
Where Dorchester Bay opens toward the outer harbor, the shoreline of South Boston curves around a three-mile arc of beaches, causeways, and promenades. This scenic stretch links the historic Fort Independence on Castle Island to the popular sands of Carson Beach, providing a continuous loop for walking, running, and cycling.
For many residents and visitors, this area is where the Harborwalk feels most like a lived waterfront, characterized by regular rituals of exercise and relaxation rather than the commercial bustle of the inner wharves.
Castle Island
- 1
Fort Independence (Castle Island)
William J Day Blvd
A star-shaped granite fort completed in the mid-19th century — the eighth defensive work to stand on this site. Anchors a 22-acre state park managed by DCR. Free volunteer-led tours from Memorial Day through Labor Day.
Echoes of 1776Known as Castle William during the colonial era, this bastion was the heavily armed headquarters for the British military command. Following the fortification of Dorchester Heights, the British realized their fleet in the harbor was completely vulnerable, forcing them to spike the fort's heavy cannons and blow up its barracks as they fled the harbor in March 1776.
- 2
Sullivan's at Castle Island
A durable landmark of the local visitor experience — a service point for food and refreshments that has become a staple for those walking the perimeter loop (open seasonally, March–October). A high glacial drumlin offering an unobstructed, sweeping tactical view over the entire inner harbor, Boston's historic core, and the seaward approaches. Maintained as a green public park and national historic site, anchoring the high ridge above the South Boston and Dorchester shorelines.
Pleasure Bay & the Strand
- 3
Pleasure Bay Causeway
A man-made lagoon shaped by a curving causeway system enclosing a sheltered swimming area, calmer than the open bay. Benches and gazebos offer places to rest.
- 4
Head Island Causeway (Sugar Bowl)
The stone breakwater ending in the small circular jetty locals call the Sugar Bowl — 360° harbor views.
- 5
- 6
Carson Beach
A high-amenity regional destination with a restored bathhouse, bocce courts, and a snack bar. Staffed with seasonal lifeguards in 2026.
Marine & Moakley Parks
- 7
Marine Park
Athletic facilities on the inland side — tennis courts, baseball fields, and the Murphy Skating Rink.
- 8
Joe Moakley Park
A 57-acre recreational hub (formerly Columbus Park) with ballfields, courts, a playground, and free parking — bridging the residential neighborhoods of South Boston with the public beaches.