At its north end is Portchester Castle, of Roman origin and the first fortress built to protect the harbour.

The narrow entrance, and the forts surrounding it gave it a considerable advantage of being virtually impregnable to attack from the sea.

[5] During the civil war parliamentary forces were able to carry out a successful cutting-out expedition within the harbour and capture the six-gunned Henrietta Marie.

In 2001 the Gunwharf Quays development, was opened on the site of HMS Vernon (a former naval shore establishment).

After improvements in the King James's and Landport Gates and the areas military defences,[10] civilian building of dockside storage and ancillary servicing facilities began from 1590.

[11] With advent of bigger steam powered ships, the physical restrictions of Camber Dock meant that it was bypassed for the larger capacity of the newer developed Portsmouth Harbour.

The sole remaining P&O service to Bilbao ceased in September 2010 but has since been replaced by Brittany Ferries as of April 2011 with two sailings a week using their modern and fast ferry Cap Finistere which can make the trip from Portsmouth to Bilbao in 24 hours with a top cruising speed of 27 knots.

On 10 May 2011 the New Terminal building at the newly renamed Portsmouth International Port' was opened by Vince Cable MP.

Most of the harbour is composed of intertidal mudflats and cordgrass marshes, and they have abundant benthic fauna which provide food for birds.

It is of national importance for dark-bellied Brent geese and for three species of waders, grey plover, black-tailed godwit and dunlin.

[13] A tunnel crossing the mouth of the harbour between Gosport and Portsmouth was proposed in 1941, which would have allowed pedestrian traffic while doubling as a subterranean air raid shelter for residents on both sides.

[14] In 1999, a study was undertaken by the Light Rail Transit Association in regard to a proposed tunnel crossing to alleviate traffic congestion in the area, particularly Gosport.

Of three construction methods examined, an immersed tube was deemed to be the most effective, as it would not necessitate closing the harbour for over a year, but rather a series of shorter closures of around 24 hours for the addition of each segment of the tunnel.

[16] The South Hampshire Rapid Transit Order 2001 provided authorisation for the project,[17] with construction intended for 2002, but financial problems delayed this.