A chain was strung from Blockhouse point to a similar tower in Portsmouth, which could be raised to prevent entry to the harbour by enemy ships.

[6] The Cowdray engraving of The Encampment of the English forces near Portsmouth in 1545 depicts all three structures, with Haselworth still under construction;[4] two years later the 1547 Inventory of Henry VIII lists 'Lymden's Bulwark by the West Haven, under Captain John Lymden' and also 'Hasillworth Castle', but the blockhouse is omitted (implying it was no longer armed at this time).

[9] An inspection in the early 18th century, however, found that the defences had fallen into poor condition; Captain Talbot Edwards (who, as Second Engineer, was made responsible for surveying the Portsmouth fortifications) said of the battery that 'this work like the rest is all gone to ruine'.

[13] The fort was surrounded by a moat on the south-west and south-east sides, the former crossed by a bridge which was protected at its far end by a stone redan and angled outer earthworks.

[10] Beyond Fort Blockhouse the Gosport fortifications were extended northwards in 1757 in order to enclose the Weevil brewery (which had been acquired by the Victualling Commissioners a few years earlier).

New defences were constructed for Gosport in 1778, with the bastioned Fort Monckton situated on the ground that had been occupied by Haselworth Castle in the Tudor era.

This created a line of bastions defending Gosport all the way from Blockhouse Point up to Forton Lake on the far side of the town, with French prisoners of war making up a part of the construction workforce.

[16] In 1813, as noted on a preserved datestone,[12] the fort began to be remodelled: the west demi-bastion was extended to the north in 1817-20 and not long afterwards the sea battery was rebuilt with thirteen covered casemates housing 32-pounder guns.

This work was part of a substantial rebuilding, which saw the creation of the north bastion: a rounded structure with casemates, built of red brick and fronted in limestone, with upper and lower batteries which provided a field of fire over the harbour.

[11] It was linked, to the west bastion on the one hand and to the sea battery on the other, by a pair of long brick-built casemated buildings: the former providing accommodation for officers,[17] the latter for other ranks.

[8] In 1873 Fort Blockhouse began to be used by the Royal Engineers as a base for the deployment of remote-controlled submarine mines as a line of defence for the harbour.

[19] At the edge of the headland, to the north of the fort, they built a short pier and several buildings over time, including a set of boathouses and cable sheds (which are still in situ as of 2024, alongside an associated slipway).

[11] In 1892 a new School of Submarine Mining was established on a site at Stokes Bay (close by Fort Gilkicker), which had previously been used for testing and experimental work.

[17] The establishment also expanded beyond the lines of the original fort on Blockhouse point: west of the main gatehouse, the defensive moat was infilled in the 1920s and the earthworks were levelled; 'Vulcan Block', providing accommodation for seamen, was built there in 1933, along with a new guardhouse and various other buildings.

At the same time, with the Submarine Service set to take on responsibility for the UK's nuclear deterrent, HMS Dolphin underwent a major expansion: a row of new large accommodation blocks was built alongside the recreation ground, to the west of the fort, looking out on the Solent.

[10] In the mid-1960s the training area around the SETT was rebuilt, with modern teaching blocks replacing the wooden huts formerly employed;[26] facilities included practice torpedo tubes, sonar sets, fire-control, missile-launch and navigation systems.

[34] After the decommissioning, control of the fort's jetties passed to the Queen's Harbour Master (who managed them on behalf of the Naval Base commander).

[37] The Royal Navy Submarine School (RNSMS) remained at Fort Blockhouse until 23 December 1999, whereupon it moved to HMS Raleigh in Cornwall.

[52] On an earlier occasion, when Charles II was sailing into Portsmouth to inspect the newly-finished fortifications in 1683, a gunner at Fort Blockhouse was killed when one of the guns burst as the salute was being fired.

[53] Since 2008 the National Coastwatch Institution (NCI) has maintained a daily lookout from the fort's signal tower;[54] it was visited by the Princess Royal in 2023.

[58] In 2015 Hornet Services Sailing Club signed a new lease on land and property at Fort Blockhouse 2 and 3, safeguarding its continued occupation of the site until at least 2065.

[61][62] Surveys were undertaken from January to March 2020 to determine which structures at the site would become listed buildings, and major job cuts took place at the end of the year.