Pendennis castle was originally built by King Henry VIII as part of a series of forts which were to guard the entrance to the river Fal.
It was built as a "device fort" which was a new type of castle designed to house and be defensible from artillery, and this new design was the reason it stood so long against Oliver Cromwell and his army.
One tactic that Cromwell was well known for was to attack a castle by setting up cannons in fortified positions just in range of the target, then he would shell the position until they either surrendered or the castle was open enough to storm into. This worked very well against all the earlier castles since they were not built with their own cannon positions and had weight bearing walls which would collapse if the bottom was damaged enough, however Pendennis castle was of a complete new design and this tactic did not work against it.
During March of 1646 Cromwell arrived at the castle and attacked, though his early attempts were not successful. This led to a siege which last 5 months where around 1000 people, including soldiers and all the nearby men, woman and children who fled into the castle for safety were trapped inside.
The castle was the last royalist castle in England to fall to Cromwell and his forces, and only had to surrender due to starvation. But after Cromwell took the castle he did not follow his usual pattern, which was to destroy the castle to point where it would be useless as a defensive structure. Instead he deemed it to important and left it as one of the handful which he did not ruin.
Today the castle is maintained by English Heritage and is open to the public all year round as a tourist attraction.
All of the ghosts that haunt the castle are believed to be from the siege of 1646, where hundreds of civilians were all trapped in with the soldiers and forced to live in cramped dirty conditions with almost starvation rations.
The castle was not prepared for a siege with so many people and only managed to last 155 days before being forced to surrender, but they held out for as long as possible and reportedly lost well over 100 people to starvation before their commander decided to give in, which was recognised as a surrender with honour by Cromwell.
There are various rooms within the castle that are believed to be the places where the bodies of those who died from starvation during the siege were kept. People have reported many strange sensations in these rooms, such as being overcome with sudden and intense emotions of fear and sadness, as well as what's been described as a very dark presence.
The lower rooms of the castle also frequently experience paranormal activity. This would have been the area the civilians were kept in during any attacks, which may explain why so many people have claimed to have experienced something here. The most common of which is strange sounds, with whispers and the faint sounds of crying coming from the area, though upon inspection nothing is ever found.