Rapid advances in military technology made the new defences obsolete before the end of the decade.
The introduction of rifled breech-loading guns, capable of firing explosive shells, and the construction of armour-plated warships left Hurst's guns and fortifications inadequate.
Fears grew in 1859 that France might invade England, potentially in a surprise attack.
A 1859 Royal Commission recommended that Hurst, as one of the key forts protecting Portsmouth, should be upgraded as a matter of priority.

Hurst was equipped with new heavy, RML guns in heavily protected casemated positions.
These weapons were slow to reload, so to ensure that the castle would have a good chance of hitting enemy ships passing at speed, two long batteries of weapons were built onto either side of the old castle, a west wing with 37 gun positions and an east wing with 24 positions.
These were built on top of the two gun batteries constructed there a few years before and also required the castle's moat to be filled in.
As work progressed, the decision was taken to reinforce the batteries with additional iron shields.

The work eventually cost £211,000; it began in 1861, with the majority of the work being completed by 1870 and the additional iron shielding by 1874.
In peacetime, the castle held around 131 officers and men but it would have needed to be substantially reinforced during wartime if all the weapons were to be manned.
Although the fort never received its full complement of guns, by 1881 it had ten 12.5 inch, 38 ton (317 mm, 39,000 kg) rifled muzzle-loading (RML) guns, fifteen 10 inch, 18 ton (254 mm, 18,000 kg) RML guns, five 9 inch, 12 ton (228 mm, 12,000 kg) RML guns, and 3 64-pounders (29 kg), together forming a powerful arsenal.

Two new lighthouses were constructed in the 1860s.
The first was the "Low Light" built into the rear wall of the west wing of the castle.
The second lighthouse was the "High Light" – the free standing Hurst Point Lighthouse built on the end of Hurst Spit between 1865 and 1867.
A new public house, the Castle Inn, was established on the north end of the spit.
A narrow-gauge railway was constructed in the 1880s to move supplies into the castle.

By the 1880s and 1890s, both the power of naval artillery and the speed of warships had further increased, leading to further investment in the castle between 1888 and 1893.
The keep's magazine, which could hold up to 2,250 shells, was reinforced with concrete and the south bastion of the old castle was filled in with shingle and concrete for additional protection.
New 6-pounder (2.7 kg) quick-firing guns were installed in a battery on the end of the east wing, supported by machine guns, to enable them to target fast moving vessels, particularly the new torpedo boats.
Further emplacements for 12-pounder (5.4 kg) guns followed, including one on the top of the keep.

Thomas Clarke, one of the executed leaders of the Easter Rising in Dublin in 1916, was born in Hurst Castle.
Rapid advances in military technology made the new defences obsolete before the end of the decade. The introduction of rifled breech-loading guns, capable of firing explosive shells, and the construction of armour-plated warships left Hurst's guns and fortifications inadequate. Fears grew in 1859 that France might invade England, potentially in a surprise attack. A 1859 Royal Commission recommended that Hurst, as one of the key forts protecting Portsmouth, should be upgraded as a matter of priority.
Hurst was equipped with new heavy, RML guns in heavily protected casemated positions. These weapons were slow to reload, so to ensure that the castle would have a good chance of hitting enemy ships passing at speed, two long batteries of weapons were built onto either side of the old castle, a west wing with 37 gun positions and an east wing with 24 positions. These were built on top of the two gun batteries constructed there a few years before and also required the castle's moat to be filled in. As work progressed, the decision was taken to reinforce the batteries with additional iron shields.
The work eventually cost £211,000; it began in 1861, with the majority of the work being completed by 1870 and the additional iron shielding by 1874. In peacetime, the castle held around 131 officers and men but it would have needed to be substantially reinforced during wartime if all the weapons were to be manned. Although the fort never received its full complement of guns, by 1881 it had ten 12.5 inch, 38 ton (317 mm, 39,000 kg) rifled muzzle-loading (RML) guns, fifteen 10 inch, 18 ton (254 mm, 18,000 kg) RML guns, five 9 inch, 12 ton (228 mm, 12,000 kg) RML guns, and 3 64-pounders (29 kg), together forming a powerful arsenal.
Two new lighthouses were constructed in the 1860s. The first was the "Low Light" built into the rear wall of the west wing of the castle. The second lighthouse was the "High Light" – the free standing Hurst Point Lighthouse built on the end of Hurst Spit between 1865 and 1867. A new public house, the Castle Inn, was established on the north end of the spit. A narrow-gauge railway was constructed in the 1880s to move supplies into the castle.
By the 1880s and 1890s, both the power of naval artillery and the speed of warships had further increased, leading to further investment in the castle between 1888 and 1893. The keep's magazine, which could hold up to 2,250 shells, was reinforced with concrete and the south bastion of the old castle was filled in with shingle and concrete for additional protection. New 6-pounder (2.7 kg) quick-firing guns were installed in a battery on the end of the east wing, supported by machine guns, to enable them to target fast moving vessels, particularly the new torpedo boats. Further emplacements for 12-pounder (5.4 kg) guns followed, including one on the top of the keep.
Thomas Clarke, one of the executed leaders of the Easter Rising in Dublin in 1916, was born in Hurst Castle.
Hurst Castle