Castle Island is South Boston's most popular beach, and along with Fort Independence, a popular day trip from the city.

Once an actual island, but now connected to the mainland after a strait was filled in 1891, Castle Island anchors Pleasure Bay Beach, the most developed of the four South Boston beaches looking out onto Dorchester Bay.
The Head Island Causeway now connects the tip of the island back to the Mainland. The beach surrounds a lovely protected cove - actually a lagoon, due to the causeway.
Famed 19th century landscape designer Frederick Law Olmstead designed Pleasure Bay Beach as a Marine Park, a jewel in his "Emerald Necklace" of parks surrounding Boston. The park includes 22 acres with some of the best views in this part of Boston.
HarborWalk extends along the beaches overlooking Dorchester Bay, and wraps around Pleasure Bay Beach. Although logically you should be able to continue along the water back up to the South Boston Waterfront, HarborWalk is not yet in place in all spots - maybe someday!
On Pleasure Bay Beach, you'll find Sullivan's (locally called "Sully's"), where you can get memorable hotdogs, burgers, and fries during the summer.
Can you swim here? People do - but I don't recommend venturing into the water due to its jellyfish population. I like it best as a place to walk, enjoy the views (and perhaps some fries), explore the fort, and soak up some sunshine. Best of all, you can get here in 15 minutes or less from the city.
Near the peak of the former island stands Fort Independence, a 5-pointed granite structure situated at the site of an earlier fort begun in 1634 and destroyed by the British on their retreat from Boston during the Revolutionary War.
I took the above photograph showing Fort Independence in February so that you can actually see it - otherwise, trees mostly hide it, although the whole area is lovely and green during late spring and summer, and breath-taking when fall foliage blazes against the granite fort.

Fort Independence prevented British naval attacks during the War of 1812 because of its strategic location. The fort that you now see is a newer structure, built between 1834 and 1851.
Boston-born fiction writer and poet Edgar Allan Poe served at Fort Independence in 1827 after lying about his age and enlisting in the U.S. Army as "Edgar A. Perry." Literary rumors cite the old fort, or a story Poe heard while stationed there, as inspiration for his famous 1846 chiller, The Cask of Amontillado.
You can walk around the fort and still see some of the mounted cannons - as well as gorgeous harbor views.
During summer months, Fort Independence opens for visitors. Explore on your own, or take a free tour conducted by park rangers.

Boston Marriott Long Wharf Hotel
