Home » Fort Mitchel, Spike Island– making a military monster

Fort Mitchel, Spike Island– making a military monster

The history of the largest military structure in Ireland…

Fort Mitchel on Spike Island Cork, showing the scale of the fortress built by the British in the early 18th century.  Cork Harbour was vitally important to defend Britain's interests right through the 17th, 18th and early 19th century.  Fort Mitchel is likely the largest military structure in Ireland, and one of the largest start shaped forts anywhere in the world.
Fort Mitchel – Spike Island

The first fortification on Spike Island arrived in 1779 while the American War of Independence raged.  American vessels were capable of crossing the Atlantic and wreaking havoc on the vital Cork Harbour supply lines, while much closer to home, France and then Spain joined the American side and took arms against their frequent foe, Britain. 

It was not unreasonable to think an invasion of Ireland could be undertaken to open up a new front in the war against Britain. In fact the event would transpire several times before the 18th century had passed.  Over the centuries, superpowers had attempted to invade England and ‘The kingdom of Ireland’ was seen as a suitable back door. 

The great sail ships of the age and the global outlook of these modern superpowers made Cork harbour a vitally important asset, with its ability to safely shelter entire fleets and provision them from the surrounding fertile countryside.  That asset had to be protected, and Spike Island was to play a central, epic role in this undertaking.

The need for defence led to one of the largest construction projects ever undertaken in Ireland and the British Empire, and resulted in the colossus that is Fort Mitchel which dominates Spike Island to this day.

This early map of Cork Harbour shows the vast expanse of one of the worlds largest natural harbours.  Cork City was an important location for British interests right up until the first world war.  Only Irish Independence deprived them of its use, a situation that frustrated Winston Churchill
An early 1700’s map of Cork Harbour, showing its vast expanse with Spike Island sitting centrally.

The first fortification on Spike Island consisted of a battery of cannons but its usefulness passed with the end of the American conflict. It was dismantled, but men like General Charles Vallancey were astute enough to know that peace was likely to be short lived. 

He lobbied for and eventually got permission for a sizable 10 acre fort named Fort Westmoreland, which was built on the island in the early 1790’s.  The designer of the fort and colorful character General Charles Vallencey was prolific in his lifetime, as an excellent architect, engineer and early Irish historian, a multi-published writer and husband to 4 wives and father to 14 children! 

The first fort on Spike Island Cork, Fort Westmorland.  Built in the early 1790's to protect Cork Harbour and the British fleet, and to defend from invasion by France who were successful in Europe at the time, particularly when Napoleon came to power.  Ten acres in size, the fort was not inconsiderable, but it would  be dwarfed by the much larger 24 acre Fort Westmorland two, now Fort Mitchel, which started construction in 1804.
The first Fort Westmorland – ten acres in size, and not inconsiderable in its ability to defend the harbour

He was well aware of the need to defend the Empire from seaborne attack, having spent time living in heavily fortified Gibraltar where he learned of the techniques used in the great European coastal defensive locations, like those at Malta. 

The first formal fort on Spike Island was not insignificant at ten acres in size, but Vallancey soon felt something even more impressive was required as a deterrent, with the emergence of Napoleon and a bitter war with France not going well. 

The population of France outnumbered Britain’s 3 to 1 at the time, and the existential threat was palpable. He pushed for the funding for further protection, and permission was given for a military behemoth that stands tall to this day.

One of the largest military structures in the world…  

The current Fort Mitchel, named Fort Westmorland when construction began in 1804.  At 24 acres it is the largest military structure in Ireland, and would later become the largest prison in the world.  There has never been a larger prison before or since.  It cost well over a billion in modern money to construct.
Fort Mitchel from the air – at 24 acres, it could hold many thousands of troops and sufficient armament to defend Britain’s prized Irish harbour.

The 24 acre star shaped Fort Mitchel that was started in 1804 is one of the largest military structures in the world.  It was the cutting edge of military technology when it was completed fifty years later, representative of centuries of advancement and refinement in military thinking and coastal defence. 

The designers chose the distinctive star shape design which had long replaced the straight square walls of Norman castles, which were big and impressive but an easy target for ever improving cannon fire.  The star shape technique emerged around the 1500’s in Italy and it was used in cities like Pisa and by Michelangelo in his defences of Florence. 

The points of the star shape, or ‘bastion’ as they are termed, meant defenders had overlapping arcs of fire all over the island, making the location one effective kill zone. 

The point of the pointed bastions was to allow for covering fire on all parts of the fort, and to have overlapping arches of fire that made the island one large kill zone.  A landing attacker would have nowhere to hide from the range of forward positioned weapons on the bastions, suffering shot and shell as they advanced.  Made obsolete by the advent of air power and more powerful Naval ships, it was the cutting edge of 400 years of military refinement in the 19th century.
The advantages of the star shaped fort design – overlapping arches of defensive fire, and nowhere for attackers to hide.

Should an enemy survive the run up the steep man made hill side known as a Glasis, and get close enough to the fort, flanking galleries were built into the sides of the bastion providing firing positions for defensive troops.  These were effectively snipers galleries, allowing defenders fire on enemy troops from relative safety. 

The whole fort itself was built with a low profile, setting it down into the islands summit in such a way that it can barely be seen by approaching enemy troops.  This made it very difficult to target the fort with cannon fire from ships in the harbour, or small arms on the ground. 

The British engineers shaved over 25 feet off the top of the island to achieve the feat, using enormous manpower, an army of one hundred horses, and in later decades, even dynamite to craft the island shape they desired!  

Well over a million Pounds was spent on the structure by 1811, close to a Billion in today’s money, and as worked continued for several more decades from 1811 to 1864, this figure was well surpassed.  This makes the structure potentially the most expensive building ever constructed in Irish history. 

The size of Fort Mitchel Spike Island is immense by every global comparison, with the roof at Wembley stadium covering 11 acres, while Spike’s fortress expands to an impressive 24 acres.  Most modern football stadiums would fit inside the forts walls twice.  In fact the whole of Alcatraz Island, not just the prison, could fit snugly inside the forts walls, while you could fit four of Rome’s Colosseum inside! 

You have to consider buildings like the Pentagon in America, with its 26000 staff and 28 acre spread, to find its equivalent

A size comparison of Spike Island Cork of southern Ireland and Alcatraz island of San Francisco of the United States of America.  At 22 acres, the entirity of Alcatraz Island could fit inside Fort Mitchel's 24 acres, highlighting the scale of the military superstructure.
A size comparison of Spike Island and its fort to Alcatraz, which could fit inside the enormous forts walls.

The vital strategic importance of Cork Harbour and Spike Island was being recognized in a fitting way, and that recognition would remain for 150 years. Winston Churchill visited the harbour as the then Head of the Admiralty and fought for its retention in the Treaty talks of 1921, holding on to the asset for British purposes.

When the island was eventually returned much later in 1938, Churchill thundered again of the areas importance, terming the forts at Cork Harbour, Berehaven and Lough Swilly asthe sentinel towers of the approaches to Western Europe”.  This time, he failed in his efforts to retain the Treaty ports.

By 1820 the main work of the walls, bastions and some accommodation blocks were complete, before the war ended and funds for military building dried up entirely.  By this time an impressive fortress had emerged fit to defend Cork harbour, unfinished and never fully armed, but undeniably fierce and if it became necessary, capable of defending an Empire.   

On its completion the Fort could comfortable garrison several thousand men indoors, and many more if tented accommodation was used in outside spaces. But its military requirement dwindled and the fortress saw use as a prison during Ireland’s famine years, holding over 2400 inmates in what became the largest prison in the world in the 1850’s. 

Spike Island Cork from above - The largest military structure in Ireland and also the largest ever prison in British and Irish history.  104 acres in size.
Spike Island from the air – dominated by Fort Mitchel.

After the prison closed in 1883 it returned to solely British military use, with Colonel Percy Fawcett a famous serving soldier.  When Ireland gained Independence in 1922 Britain insisted on retaining the vital fort as part of the peace Treaty. 

It was 1938 before the island and fort was given back to Ireland, as part of negotiations in a trade war between the two countries, and Éamon de Valera watched as the Irish  tricolour was raised for the first ever time over the fort.  The Fort was renamed from Fort Westmoreland to Fort Mitchel following Irish Independence, after the Irish Nationalist John Mitchel who was a prisoner on Spike Island in 1848.  

Today visitors can go through the forts impressive entrance walls to the enormous parade ground inside, and many of the original buildings and tunnels can be explored as part of a visit to Spike Island Cork, the epic island fortress and prison that protects Ireland’s Celtic sea. 


For the complete history of the fortificaiton of Cork Harbour and Spike Island see: Cal McCarthy – Cork Harbour – 2019, Merrion Press

John Harnett McEnery – Fortress Ireland: The Story of the Irish Coastal Forts and the River Shannon Defence Line – 2006, Wordwell

The Journal of the Royal Society of Antiquaries of Ireland – General Charles Vallancey 1725-1812 – 1993

Daniel MacCannell – Coastal Defenses of the British Empire in the revolutionary and Napoleonic Wars – 2021 – Pen and Sword Military

Paul M Kerrigan – Castles and Fortifications in Ireland 1485 – 1945 – 1995, Collins Press

Niall Brunicardi – Haulbowline Spike and Rocky Island – 1968, Cork historical guides

Other sources – Pues Occurrences / National Archives Ireland / National Archives UK

Content may be quoted with credit and link. References in upcoming book or contact me, I love to discuss the subject. For further reading and the full detailed account on the fortification of Cork Harbour, the reader is referred to Cal McCarthy's book 'Cork Harbour', Merrion Press.