The gaol is a largely wood frame structure that has had a long evolutionary history of growth and change, and presently appears as a 1-1/2 story gambrel-roofed structure that is partial clapboarded, with sections of stone wall.

The oldest portion of the building, dating to 1719 or 1720, is a stone cell block containing a single cell, with stone walls 2.5 feet (0.76 m) thick lined on the inside with oak planking.

About 1736 several first-floor chambers were added, including a kitchen, dining room, and parlor space that has a folding partition wall (added late in the 19th century), so that it be divided into two separate rooms.

Around the turn of the 19th century a second stone cell was added to the first floor, along with an extension including a dining room, parlor, and bedchamber for the warden.

After 1879 the building was used as a school, warehouse, and boarding house until 1900 when it opened as a colonial museum of relics.