While on Vacation in Scotland a few years ago, we visited several castles, in varying degrees of decay. We visited nearby Stirling Castle, but only from a distance due to it being extremely busy, but Doune is the first one we got to tour. Doune is a medieval castle with a storied history, both royally as well as militarily. Mary, Queen of Scots actually visited this castle on many occasions and you can even see the rooms she occupied. The completeness of the site helps to make it one of Scotland's most cherished castles.
The other claim to fame for Doune is the fact that much of The Holy Grail was filmed there using different parts of the castle to represent other castles. There is a self-guided tour you can take using a device with headphones and narrated by Terry Jones of Monty Python.
