There were 3 main priorities.
a) Protect London - if this fell, England fell with it
b) Protect the main Naval bases - any invasion would need constant supplies, keep the fleet in being and the invaders could be starved of supplies.
c) The Isle of Wight. This is not so obvious, but was a historical worry. Ever since the days of the Spanish armada there had been the fear that a hostile power would not invade the homeland, but seize the Isle of Wight as both a bargaining chip, and to disrupt the Royal Navy base at Portsmouth. Virtually defenceless, the IOW would require a much less imposing force to conquer, and was a much more plausible proposition.
Between 1855 and 1870 12 separate defences were built around the island. If these could be overcome, the last defence on the east side of the island, the final citadel, was designed to be the Fort at Bembridge.
Fort Bembridge was constructed on the high ground between the town of Sandown and the village of Bembridge, on Culver Down, at 320 ft above sea level the highest point on this side of the island. It was built between 1862 and 1867, at a cost of nearly £50,000.
Bembridge Fort from above. A brick built hexagonal structure set into the chalk hillside with a cannon at each corner, and parapets all round. The exit is across a drawbridge to the road on the bottom of the picture.
Bembridge Fort had 3 main functions.
a) It was the barracks for two nearby batteries covering the coast.
i) Redcliff - three 7" rifled breech loading (RBL) guns covering Sandown Bay and one facing towards Culver cliff, 100ft up a cliff and defended on the land side by a deep ditch.
ii) Yaverland - 8x 7" RBL guns facing to sea. 100ft up but the cliff was climbable and so had infantry defences and a ditch on both seaward and landward sides. Note the reason for so many 7" RBL guns was that the navy didn't want them! Their performance had been less than satisfactory at sea and a rearmament program was taking place.
b) To secure the high ground behind the Sandown Bay beach defences.
c) If the defences at Sandown Bay were stormed, or successful landings were made elsewhere on the island, Fort Bembridge was to hold out against the invader.
Bembridge had a peacetime complement of 1 officer, 5 NCOs and 78 men. Water was pumped from a well at Yaverland and stored in a 22,000 gallon tank, making the fort self sufficient in water at least.
Men of the Royal Artillery (New York Public Library)
In 1871 the fort was manned by the 103rd regiment of the Royal Artillery, and from 1872-1874 the 7th Brigade.
Attacking Fort Bembridge
So if Napoleon III had successfully landed troops on the Isle of Wight, what would his blue coated infantry have encountered as they prepared to storm Fort Bembridge?
The view up the hill towards the fort, the small mound at the top
Cliffs on the seaward side and it's position on the highest point on this side of the island meant any attack would be up hill. Also being dug deep into the chalk, and the use of bomb proof shelters gave as much protection from artillery as possible. Each corner of the hexagon had a 7" RBL cannon and the fort was well supplied with over 2,000 rounds, and 1,200 shells, whilst all the time attackers would be under fire from infantry manning the parapets.
Any enemy reaching the walls would have to cross a deep dry moat. the single entrance crossed only via a draw bridge. If they entered the moat they would be in a crossfire from the caponiers, brick built covered walkways crossing the ditch. If the French could scale the inner wall and take the central parade ground then, well, all was probably lost.
Thankfully, Prussia put paid to Napoleon and his plans, (although causing other worries!) and the fort was never needed.