Lord Salisbury described Malta, Gibraltar, Bermuda, and Halifax as Imperial fortresses at the 1887 Colonial Conference, though by that point they had been so designated for decades. Later historians have also given the title "imperial fortress" to St. Helena and Mauritius, despite their lacking naval dockyards and not serving as home bases for station naval squadrons.

The fortresses provided safe harbours; coal stores; and dockyards to protect and supply Royal Navy warships. They had numbers of soldiers sufficient not only for local defence, but also to provide expeditionary forces to work with the Royal Navy, as well as stockpiles of military supplies.

Although the War Office took over from local officials the funding and operational control of auxiliary forces in the British Isles from 1871 onwards, the trend of Imperial defence policy during the course of the 19th century was to remove regular units of the British Army from colonial garrison duty wherever strategic concerns did not require their retention (this included disbanding colonial regular units of the British Army, other than the West India Regiment and the local-service sub-unit of the Royal Artillery that would ultimately be titled the Royal Malta Artillery, though others would later be raised), with local governments expected to organise and fund auxiliary forces for local defence (although these forces would ultimately be controlled by the Imperial government via colonial Governors, most of whom were civilians, acting as military commanders-in-chief). The main exceptions were the four Imperial fortresses, which the regular army continued to garrison, and within which the part-time militia and volunteer units were funded as parts of the British Army and brigaded with regular units. The 1887 Colonial Conference sat in London from April 4 until May 9, 1887. At the conference, it was asserted that:

In order that the Royal Navy might in practice be ubiquitous, it was essential that certain bases and coaling stations should be provided with shore defences. "In addition to the Imperial fortresses Malta, Gibraltar, Bermuda, and Halifax, it would seem necessary to defend on an adequate scale, Cape Town and Simon's Bay, St. Helena, Sierra Leone, Port Louis (Mauritius), Aden, Colombo (Ceylon), Singapore, Hong Kong, Port Royal (Jamaica), Port Castries (St. Lucia), and Esquimalt, in addition to minor coaling stations . . ." The imperial fortresses would remain a responsibility of the United Kingdom; but in the case of certain colonies in which local as well as imperial interests seemed to require that naval bases be maintained, the government of the United Kingdom thought that the cost should be shared..

Halifax and Bermuda controlled the transatlantic sea lanes between North America and Europe, and were placed to dominate the Atlantic seaboard of the United States (as demonstrated during the American War of 1812, when the squadron of the Royal Navy's North America Station maintained a blockade of the Atlantic coast of the United States and launched the Chesapeake Campaign from Bermuda, defeating American forces at Bladensburg, burning Washington, DC, and raiding Alexandria, Virginia, before ultimately being defeated at Baltimore and forced to withdraw back to Bermuda), as well as to control the western Atlantic Ocean from the Arctic to the West Indies. In 1828, Royal Navy Purser Richard Cotter wrote of Bermuda:

The possession of Bermuda, as the key of all our Western Colonies, is of the first importance to England, for if a foe of any maritime strength had possession of it, our trade would be exposed to much annoyance, if not total destruction.

Gibraltar controlled passage between the Atlantic Ocean and the Mediterranean Sea, and Malta, aside from supporting operations in the Mediterranean and Black Sea, served as a base for naval and military forces that would be able to deploy relatively quickly to the Indian and Pacific Oceans once the Suez Canal was completed in 1869.

Halifax ceased to be an Imperial fortress in stages. With the 1867 confederation of the Dominion of Canada (under which all of the colonies of the British Empire's administrative region of British North America, except Bermuda and Newfoundland, were "federally united into One Dominion under the Crown of the United Kingdom of Great Britain and Ireland, with a Constitution similar in Principle to that of the United Kingdom ..."), military defence of Canada would be transferred to the militia of the dominion government, and the British Army withdrew most of its establishment from the continent, leaving small military garrisons to defend the Royal Naval Dockyard at Halifax, Nova Scotia and the Esquimalt Royal Navy Dockyard in British Columbia. These garrisons were withdrawn along with the Royal Navy establishments when the two Canadian dockyards were closed in 1905, then sold to the government of the dominion.