March 1643. From his quarters in Castle Cornet, the Lieutenant-Governor of Guernsey ordered his troops to fire on the island itself. Two hundred yards offshore at the entrance to St.Peter Port harbour, the castle ws now isolated in more ways than one. A Royalist facing a population which supported Parliament and had called on him to surrender, Sir Peter Osborne knew he was in deep trouble. The English Civil War, in which parliament rose up against Charles I and which led to his execution, had reached the island.
So began a siege that would last into the next decade, one more episode in the castle's long history of embattlement, usually featuring the French.
In 1643, though, it was Briton against Briton and the scene was set for a test of patience and determination as much as courage. The Royalists were unable to do much about the landbased opposition and the Parliamentarians were apparently not equipped to storm the castle. Nor were they inclined to, after one disastrous attempt which ended with the raiding party stranded by the rising tide on the rocks at the foot of the walls.
The Guernsey people were unhappy with the way Osborne had spent money intended for fortification of the castle -  and the fact that he taxed them to supplement it. Osborne was unpopular anyway because he was aloof and kept himself to himself in the castle. When he had to set foot on Guernsey soil to leave or enter the island he kept a low profile and traveled by quiet routes.
Castle Cornet was supplied throughout the Civil War siege by ships sent by the Lieutenant - Governor of Jersey, George Carteret, although this arrangement was rather haphazard. Osborne was by all accounts almost universally unpopular, disliked even by his own side. This included Carteret, so supplies and fresh troops were sent when his Jersey counterpart felt like it, rather than when Osborne felt the need.
Early in the siege, the Royalists struck a psychological blow by kidnapping three prominent local officials. Jurats de Beauvoir, Carey and de Havilland were tricked into boarding an enemy ship at Fermain Bay, thinking it belonged to their own side. A few days earlier they would have been right, but now the captain has switched his allegiance to the Crown. He promptly took them to the castle, where they were imprisoned.
After 43 days the three managed to make a hole in the floor and dropped through to the cell below, where they found a large quantity of flax. Using this to make ropes, they clambered down the outside wall on the very morning appointed for their execution and fled across the rocks to St. Peter Port.
Osborne's uncomfortable relationship with the influential Carteret resulted in the former's removal from office in 1646, a move which in the light of the way things were going, he may well have regarded as a mixed blessing. It certainly wasn't much of an improvement for Carteret, since he found himself similarly disenchanted with Osborne's successor Sir Baldwin Wake.
Carteret used his friendship with the newly-crowned Charles II to have Wake ousted in 1649 and for the final two years of the siege, Castle Cornet was commanded by Col. Sir Roger Burgess.
The castle holds the distinction of being the last Royalist stronghold to surrender. The end came on 19th December 1651, shortly after the fall of Jersey to the Parliamentarians and a naval blockade which meant no more supplies or reinforcements for Burgess. In recognition of their humane treatment of prisoners, Burgess and his men were allowed to leave at the end of the siege with honour, "drums beating, ensigns displayed."