Craigmillar Castle, located in Edinburgh, Scotland, is a cool medieval ruin that once hosted Mary, Queen of Scots. Built in the 14th century by the Preston family, it has a tower house with a great hall, prison, and even a room where Mary stayed. The castle was damaged during the Burning of Edinburgh in 1544 but was repaired and later became the setting for the infamous 'Craigmillar Bond' plot to murder Mary's husband. Today, visitors can explore the castle's maze of tunnels and check out the awesome views of Edinburgh from the tower house. It's like stepping back in time and walking in the footsteps of royalty!