Richard the Lionheart's chateau is obviously well rated in guidebooks of France sold in Belguim, Germany and particularly the UK - this is apparent from the high percentage of cars registered in these countries that one finds in the parking lot. Proportionally, not that many French vehicles. Living in Northern France, we do not consult such guidebooks. My wife and I thus stumbled across Chateau Gaillard by chance one fine and sunny day a few years ago when we were looking for somewhere to spend a few hours walking along the Seine River. Since then, it has become a "must see" whenever we have people visiting us for more than a couple of days. Not well known, it always impresses our visitors, who have never heard of it. The partially ruined castle is historically interesting, but the most impressive view is from the hillock just above the parking lot - the meandering Seine with the ruin in the foreground. Entrance fee to visit the Castle Keep - but that is not that interesting except for the explanation inside of how they dug the well - have only visited the Keep once. The visit of the outer ruins is free, however and you get a great view of the river from there.
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