The terms are used differently in English and American. In Aquitaine, the only acceptable dialect is English.

In English - the language of most anglophone tourists and virtually all anglophone residents in rural France - a castle is almost always a medieval building still showing signs of fortification. The few exceptions are for buildings, mostly in the British Isles, where either the name's either been applied, or the building's been rebuilt in the past few centuries, the fortifications removed, but the name (as in Sudely Castle) retained .

A chateau - again in English - is a word used only to refer to buildings in France, or the French-speaking territories around it. It's used almost identically to the English "stately home" or some uses of "country house", and a chateau must look primarily residential. Most English speakers imagine the term refers to relatively recent buildings (post 1500 being the usual English criterion for "relatively recent"), though applying it to a building from the Romanesque era would be OK if it looked residential.

I'll leave it to Ameican posters to describe how their language is used. But, whereas in English the distinction between chateau and castle is clear and consistently used by all speakers (so misusing these terms will confuse English speakers), I suspect most users of the American dialect are a lot vaguer. You sometimes find Americans using "castle" as the generic term for both chateaux and castles, which will simply baffle an English speaker.

Whereas in Paris, and bits of the Riviera, there's an argument that the dominant dialect might be seen as American, in Aquitaine, and in your case, there's simply no debate. You speak English, and in English the Chateau de Carlux (that's what it's called in English) is a castle. Guide books in English should go "The Chateau de Carlux was built in xxxx. The castle's typical of those erected during the xxxx wars" or whatever.

The distinction's subtle, and the French often get it wrong. But it sticks out like a sore thumb to an English speaker when they do - like translating "sensible" as "sensible"

I wrote a trip report in which I tried to distinguish the difference between what Americans might consider a château to look like and what they might consider a castle to look like (even though these terms are obviously interchangeable):

"I'm American and being an American I often tell my French wife that I know what a castle looks like. I've seen them in movies, and in Disneyland. Cinderella and Sleeping Beauty both lived in castles. I am a castle expert. As any castle expert knows, castles have spires, towers and turrets and are surrounded by a moat, preferably filled with water and teeming with piranhas, sharks and alligators, similar to the way these creatures enjoy living together in The Great Lakes. Castles do not resemble big houses, like Versailles."

To make things simpler, we can all just switch to speaking French whereby all of these buildings will be called châteaux, although when the walls are a meter thick we might prefer to use the term <i>château-fort</i> or <i>château fortifié</i>.

Let me just add that it doesn't really matter whether you call it a château or a castle. As long as you provide a photo then, well, a picture is worth a thousand words (or in this case two words - château and castle-).

Thanks for your input. I'm trying to put together a response, and had a look at the official site for Beynac, home of one of the largest Chateaux-forts.

It starts off saying - in French and on the English site:
Beynac,' un village un castel, un fleuve' - just to make things more interesting.

Then it goes on - meaning what?
On this ship of limestone failed at feet of the river Dordogne, pages of troubles of a fortress and its village are going to be engraved for ever … "
" … You interminable Dordogne or river? River Dordognia Monseigneur! "

So anything I do will be better than this (I hope)

Strangely enough, I agree with nytraveler and flanneruk. (I'm an American speaking American English.) Sounds like what you have in Carlux is a castle. To me, chateau implies a more complete building with no defensive aspect, one purely for luxurious living.

When we went to Gisors, I was a bit disappointed to see the chateau there. It was not what I had expected.

I've been frustrated with the French language wherein the word château can mean anything from a large, elaborate palace to a castle or a large or even not so large country house or a medieval fortress or a pile of stones. How does a palais differ from a château?

We English-speakers have benefited from the addition of French words to our vocabulary, giving us a larger pool of word choices with fine distinctions. For instance, I would differentiate between cemetery (from cimetière) and graveyard.

As for what flanneruk calls the American "dialect,", over the years I've found the British speaking more and more like Americans. It's almost disappointing. Must be the influence of TV and movies.

I have seen and read of many big (usually country) houses/mansions that are callaed "château ...", often of wine makers, with no aspect of fortress so I tend to agree with nytraveler, flanneuruk, Mimar and Dukey...

I have seen and read of many big (usually country) houses/mansions that are callaed "château ...", often of wine makers,

In the case of wine-makers, it has nothing to do with the size of the house/mansion. It is a way to designate a Grand Cru (usually a Bordeaux wine).
A château is also the upper structure of a ship deck - again, nothing to do with a mansion.

I also agree with NYTraveler and Flanneruk. I also think FrenchMystiqueTours' is perfect, from this American's point of view. When I think of "castle" I think of moats and turrets and arrow slits. I think of Provins or Vincennes. No way I'd think of Villandry as a castle. I think the term "castle" fits Carlux just fine.

On this ship of limestone failed at feet of the river Dordogne, pages of troubles of a fortress and its village are going to be engraved for ever … "
" … You interminable Dordogne or river? River Dordognia Monseigneur! ">>

lol, Carlux - a wonderful example of franglais.

it does highlight something that i have noticed which is that in this sort of literature, french people tend to use rather more highflown language than we would in the same sort of publication.

ASlso, someone (Who doesnt speak a word of Engolish) puts it through a translation program,and accepts whatever comes out.

I was at the Grotte de Maxange several years ago, and said to the very nice young man who had given us the tour in French that the English captions that accompanied the photos weren't comprehensible by English speakers - 'yes they are' he said. When I pointed out - very politely, and in French - that in fact they weren't, he said 'I did them, and they are good enough for the English.' End of conversation, and unfortunately representative of the opinion of a number of people here. Luckily since I speak French I could read the French captions and figure out what they were trying to say.