On that side it is sheltered by a huge breakwater, over 2 m.
East of the breakwater and parallel to it for 2700 ft.
The harbour has an artificial breakwater and extensive modern fortifications (Fort Preble, on the Cape Shore; Fort Levett, on Cushing's Island; Fort Williams, at Portland Head; and Fort McKinley, on Great Diamond Island) among the best equipped in the United States.
The tidal harbour, which is owned by a company, is enclosed by two piers and a breakwater, the area being about 30 acres, and the quayage 1400 yds.
A breakwater and sea-wall prevent the blocking of the harbour entrance and encroachments of the sea; and there is another breakwater at Landguard Point on the opposite (Suffolk) shore of the estuary.