When I read the question yesterday afternoon, I had no idea why it was called Castle Square. The only thing I knew was that Castle Square Hotel and Castle Square Theatre were located on one corner of the intersection of Ferdinand (now Arlington), Tremont, West Castle (now Herald), and Chandler Streets. The hotel and the theatre stood on most of the Berkeley, Chandler, and Tremont Street block from the late nineteenth century until 1933, when they were demolished. The Chandler Inn is all that remains of the old complex.
Since the first mention of anything “castle” that I knew of in that area was West Castle Street, I looked at some old maps and city directories to try and find when that street name first appeared. West Castle Street was the name of what is now Herald Street, but only the section that runs north west from Washington Street. An 1874 ward map confirms this. Ward maps from 1857 and 1865 list a West Castle Street, north west of Washington, and an East Castle Street, south east of Washington Street. Sometime between 1874 and 1883, the names of the streets change from West and East Castle Streets to Castle and Motte Streets, respectively. The 1885 map from King's Handbook of Boston (at left) shows Castle and Motte Streets. The orange arrow points to Castle Street and the gold star marks the location of the Castle Square Hotel and Theatre. Ward maps until at least 1938 label the streets as Castle and Motte Streets. The first mention of Herald Street I found was in a 1952 newspaper article, so sometime between 1938 and 1952, Castle and Motte Streets became Herald Street. In the mid-1960s, buildings near Herald, Paul, Albion, Village, Emerald, and Middlesex streets were razed. The Castle Square housing complex was then built on the site. I assume that the 1960s Castle Square project was named Castle Square because of the former hotel and theatre nearby and/or the former name of Herald Street.
So the street was named Castle Street and the hotel and theatre were named Castle Square. I assume that this is where the name of the current Castle Square housing complex comes from. But why did those nineteenth century Bostonians use the name “castle” for the hotel, the theatre, and the street in the first place?
As most South Enders and Bostonians probably know, the intersection of East Berkeley (formerly Dover) and Washington Streets marks the approximate location of the narrowest part of the Boston Neck, the thin piece of land connecting Boston’s Shawmut Peninsula with the mainland. Early colonial Bostonians built fortifications on the Neck as early as the mid-seventeenth century. A map from 1775 shows a fortification on the Neck and a short street or path along its border labeled Castle Street. The Hale map from 1814 shows a Castle Street in the same location, near the edge of where the late eighteenth century fortification was located. The nineteenth and twentieth century pre-Herald street Castle Street sits along this same late eighteenth and early nineteenth century Castle Street.
I cannot be one hundred percent sure why early Bostonians named it Castle Street, but I think it may be because sometimes forts were described as castles. Castle Island (a.k.a. Fort Independence) is a good example. So it’s possible that the street took the name Castle Street because it was located along the line of a Boston Neck fortification and the name stuck all the way until the present day. If my speculation contains any truth, the name Castle Square is an interesting descendant of early Boston.