La Roche-Guyon is located in the département of the Val d'Oise (95), just 66 kms north-west of Paris. La Roche-Guyon has two châteaux, one situated atop the chalk cliff that's basically nothing more than a ruin (the dungeon dates back from the 12th century), and another one, the main château, at the base of the cliff that's still in much nicer condition (its construction started in the 13th century).
In the 18th century, the château, owned by the La Rochefoucauld family since 1659, underwent a fair amount of renovation in order to make it less of a medieval fortress and more of a residence fitted for aristocrats.
After the revolution, the dungeon's height was reduced by a third. It was supposed to be destroyed just in case it were to be taken by the counter-revolutionaries. Razing castle dungeons and/or towers was a common practice back then.
Near the end of World War II, the château became the headquarters of Rommel, assigned to the defense of the French Atlantic coast against a possible allied invasion. Two months after D-Day and even though the Germans were long gone, La Roche-Guyon was bombarded and the château took a few hits.
The château is featured in a "bande dessinée" (comic strip) called "Le Piège diabolique" by Edgar P. Jacobs (part of the "Blake et Mortimer" series). See Bande Dessinée on this very blog for more info on this very popular form of story telling.
The château can be visited almost year round (it is closed in the winter, from late november to early february). For more info, please visit the château de La Roche-Guyon website.