Based on "Isle of Children", a flop Broadway play that ran two weeks in 1962. Jodie Foster's role was originally played on-stage by Patty Duke.
Richard Harris thought the script was faulty, and threatened director Don Taylor with a good thrashing. He also locked William Windom in his dressing room for several hours.
As of 2016, the Mahone Bay house, in which this movie was filmed, is still standing, and looks more or less the same as it did in the 1970s. However, Dierdre's wooden castle is no longer there. The house is now sold as cottage property, and gets rented out to visitors throughout the summer.
Originally called "The Last Castle", that is also the title of the theme song, written and sung by Richard Harris.
During the end credits of the film, the flag flapping in the wind atop Dierdre's wooden castle, is the flag for the province of Nova Scotia, where the film takes place.