We went to Blackness Castle one day. It sits at the end of a promontory in the "Firth of Fourth," which means the sea inlet into the river Fourth.
It was very cool and very cold! The Castle was often used as a prison for important political or royal enemies, much as the Tower of London was used. Blackness was almost impossible to capture.
If you have seen "The Outlaw King" or "Outlander" the castle is used in the movie and TV show.