Guédelon castle is a big castle that people are building in France. They are using the same tools, techniques and materials that they would have used a really long time ago – like hundreds of years ago! It's like they are building a castle as if time stopped all those years ago and they just started building again now.
They made a plan for the castle, just like people make a plan for a new house. They dig up rocks and earth from the ground to get the materials to build with. They make the bricks from clay and put them in a really hot oven called a kiln to make them hard. They use big heavy stones to build the walls, and they use wooden pegs and joints to hold everything together. They even made their own ropes and used them to move the big heavy stones around.
Lots of people work on the castle at the same time, and they all have different jobs to do. Some of them are working on cutting stones into the right shape, some of them are moving materials around, and some of them are using big tools to shape the wood.
It takes a really long time to build a castle like this, and it's not finished yet. But when it's done, it will be a castle just like the ones people made all those years ago, and it will be really amazing to visit and look at!