In 1803, war with France appeared imminent once again.
After some discussions, it was agreed to adapt the 16th-century keep to enable it to hold six 24-pounder (10.8 kg) guns; the roof was vaulted and a central stone pillar was installed to run up through the building, work estimated at the time to be likely to cost £4,122.
The historian Andrew Saunders likens the resulting building to the various Martello towers being constructed along the south coast at this time.
It was proposed to build two temporary gun batteries to replace the 1795 emplacements, which had suffered from the salt air and decayed, but the plan was turned down in order to focus attention on the redevelopment of the keep.
The work was carried out by a mixture of soldiers and civilian contractors and was completed by the end of 1806.

The remodelled castle did not play an active part in the Napoleonic Wars, although it was used in 1809 as a hospital for injured soldiers returning from the Peninsula Campaign in Spain.
A second lighthouse, known as the High Lighthouse, was constructed alongside the castle in 1812.

Official interest in Hurst Castle increased again in the 1840s, as the introduction of shell guns and steam ships created a new risk that the French might successfully attack along the south coast.
Previously, sailing ships had been only able to pass the castle slowly when moving against the tide, making them vulnerable to its guns; steam ships threatened to cruise past at speed.
Military estimates and surveys in 1850 and 1851 suggested that the armament should therefore be significantly increased, to include more and much heavier guns.

Improvements were carried out between 1852 and 1856 at a cost of over £6,725.
The keep was adapted to support 32-pounder (14.5 kg) guns, the seaward-facing bastions and curtain walls were reinforced with brick casemates and new gun positions, and the moat was deepened to protect against any surprise attack.
Two batteries were built to the west and east of the castle, protected by shingle and earth and linked to the old castle by covered passageways called caponiers; a defensible barrack block was built beside the western battery, to provide further protection against any attack from the land.
The result was that Hurst was re-established as a powerful fortification, equipped with fourteen 32-pounder guns, fifteen 8-inch (203 mm) shell guns and two 32-pounder carronades; in wartime, these required a team of 440 men, with a peacetime garrison of 105 soldiers.
Fort Albert was built across the water on the Isle of Wight to provide supporting fire.
The old inn was first reused as the garrison canteen and then demolished.

The 1852, the Isle of Wight Electric Company laid a specially-armoured telegraph cable to link the castle with Keyhaven on the mainland and Sconce Point on the island; two years later, the Electric Telegraph Company linked the castle to Southampton.
Hurst used flags to pass on telegraphic messages to and from shipping entering the Solent.
In 1803, war with France appeared imminent once again. After some discussions, it was agreed to adapt the 16th-century keep to enable it to hold six 24-pounder (10.8 kg) guns; the roof was vaulted and a central stone pillar was installed to run up through the building, work estimated at the time to be likely to cost £4,122. The historian Andrew Saunders likens the resulting building to the various Martello towers being constructed along the south coast at this time. It was proposed to build two temporary gun batteries to replace the 1795 emplacements, which had suffered from the salt air and decayed, but the plan was turned down in order to focus attention on the redevelopment of the keep. The work was carried out by a mixture of soldiers and civilian contractors and was completed by the end of 1806.
The remodelled castle did not play an active part in the Napoleonic Wars, although it was used in 1809 as a hospital for injured soldiers returning from the Peninsula Campaign in Spain. A second lighthouse, known as the High Lighthouse, was constructed alongside the castle in 1812.
Official interest in Hurst Castle increased again in the 1840s, as the introduction of shell guns and steam ships created a new risk that the French might successfully attack along the south coast. Previously, sailing ships had been only able to pass the castle slowly when moving against the tide, making them vulnerable to its guns; steam ships threatened to cruise past at speed. Military estimates and surveys in 1850 and 1851 suggested that the armament should therefore be significantly increased, to include more and much heavier guns.
Improvements were carried out between 1852 and 1856 at a cost of over £6,725. The keep was adapted to support 32-pounder (14.5 kg) guns, the seaward-facing bastions and curtain walls were reinforced with brick casemates and new gun positions, and the moat was deepened to protect against any surprise attack. Two batteries were built to the west and east of the castle, protected by shingle and earth and linked to the old castle by covered passageways called caponiers; a defensible barrack block was built beside the western battery, to provide further protection against any attack from the land. The result was that Hurst was re-established as a powerful fortification, equipped with fourteen 32-pounder guns, fifteen 8-inch (203 mm) shell guns and two 32-pounder carronades; in wartime, these required a team of 440 men, with a peacetime garrison of 105 soldiers. Fort Albert was built across the water on the Isle of Wight to provide supporting fire. The old inn was first reused as the garrison canteen and then demolished.
The 1852, the Isle of Wight Electric Company laid a specially-armoured telegraph cable to link the castle with Keyhaven on the mainland and Sconce Point on the island; two years later, the Electric Telegraph Company linked the castle to Southampton. Hurst used flags to pass on telegraphic messages to and from shipping entering the Solent.
Hurst Castle