The answer's Doune Castle in Scotland. It served as both Castle Leoch in Outlander and was visited by King Arthur in Monty Python and the Holy Grail.
I was lucky enough to have a look around last week and wanted to share a little bit of the experience with you. There was an audiotour and Sam Heughan gave some behind the scenes info about filming Outlander which was pretty cool!
Here's how the entrance to the castle looked in episode one of the first season. Next to it is how it looked on the day I visited.
Here's how it looked in 1743. Note the CGI adjusting the roof and chimney!
Inside the courtyard is medieval stonework that had to be protected during filming. Tarpaulin was laid down and then fifty tonnes of mud were barrowed in to create the authentic look needed for filming. Smoke pellets were used to create mist and apparently the staff in the castle office were smoked out!
The kitchen, domain of Mrs. Fitzgibbons, and the Great Hall where meals were served.
It was great to see around a medieval castle. I found out many things I didn't know. The Duke and Duchess slept in different rooms in the Middle Ages, the Duchess sleeping in the same room as her ladies in waiting. The Duke had a private toilet in his bedroom but the Duchess had to share with the other castle occupants. The Duke also had a private toilet in the Great Hall, and a trapdoor that led directly to his private wine supply.
The things you find out by exploring! In a week or so I'll be going up to look around Caerlaverock, site of a siege in the reign of Edward I. More photos to come soon!