Although it looks like it could have been on the hillside for centuries, this castle was actually built by hand in just the last three years.
Former soldier Mike Allen began building the castle in South Wales in 2019 as a way of helping his mental health.
After collecting stone and fallen trees from the local area, he constructed it with the help of other veterans and stone specialists.
Now Mike runs activities there and the new Welsh castle attracts visitors of all ages.
As a soldier, Mike Allen served in Afghanistan.
When he returned to Wales he suffered from depression and had flashbacks.
He was later diagnosed with post-traumatic stress disorder (PTSD) because of his experience in the war.
He says being outside and building helped him to cope with his PTSD.
The local farmer who owns the land in the Gwent mountains, gave Mike permission to build there.
He collected 100 tonnes of stone for the fort which was constructed using a method called dry stone walling.
Dry stone walls are an old way of building where stones are put together without mortar or cement to stick them together.
Dry stone walls can last hundreds of years and Mike says putting one together is like putting together a big jigsaw. The castle has lots of other features including a chessboard which Mike built using rocks from the river and black and white tiles. Thousands of people have visited the castle since it was finished, including children who live locally:
"They really take an ownership of the place," Mike told Newsround "They like getting their hands dirty, getting stuck in and helping out."