In the 18th century, the only land route into Boston was across a narrow strip of land - called "Boston Neck".
One often talks about the heart of Africa, of course, or the bosom of Europe. How wonderful that Boston gets a neck.
Fortifications were built across the neck to protect the city from a land attack, I suppose a bit like a heavy necklace, or a handsome neck brace.
When the Siege of Boston began in 1775, Governor Gage ordered that the fortifications be expanded and strengthened - they became known as "Gage's Lines".
Starting in the 19th century, land was added to Boston Neck, widening it to make room for the city to grow - and, presumably, to ease traffic on and off the island, since Boston's no longer in danger of attack from the countryside.