The Sally Port! From a plaque adhered therein:

In its original state, iron-studded oak doors stood at both ends of the sally port.
Large granite blocks, quarried in Quincy, Massachusetts, line its walls.
Note the mortar mixture of oyster shells and horsehair which has survived intact as a result of this protected location.


Now, sure, a mixture of oyster shells and horsehair sure sounds gross, bit it's not like it was dripping horsebits or anything.