This museum houses the remains of an early-18th-century cannon battery, one of several fortifications the Dutch built around Table Bay. Although it had been partly demolished and covered over during the construction of the docks in 1860, an excavation of the site in 1999 revealed the remains. You can walk around the entire site and get a good feel for what it would have originally been like.
Historical walking tours of the Waterfront start from here (minimum of four people).