"an old castle with a keep, walls, battlements, mabye a dungeon."

OK; I'm back to Scotland and all the ones I mentioned about (though some of the tower houses don't really have battlements and stuff but are really just fortified houses). Try Fyvie; Glamis; Huntingtower as well as those mentioned above:)

Germany-Burg Eltz
North Wales-Caernarfon, Conwy, Harlech, Beamuaris, Criccith
Spain/Portugal-Marvao

Chillon and Gruyere, in Southern Switzerland.

I like the Castle of Bouillon, Belgium, espeically the underground dungeons.
Anyway, have a look at http://www.castles.org/castles/

The ones in the Dordogne aren't too shabby.

Don't know where you are going, so, I'll cover a few that I have enjoyed.
Oft mentioned, but another vote anyway, Berg Elze on (off?) the Mosel in Germany. My absolute favorite.
Edinburgh in Scotland;everything you have asked for in a castle.
Stirling in Scotland fits your description and you will have a chance to see the restoration that is going on inside.
Chateau de Beynac in the Dordogne, France. Beautiful hilltop location and a real pain to the British during the Hundred Years War.
I know I'll get grief, but I loved Carcassonne in France, especially in the early morning or in the evening when the ramparts were deserted. I kept wanting to unsheath my sword.
Sarospatak Var in Hungary (Northeast of Eger); this 11th century castle has been wonderfully refurbished and maintained. There is an excellent 16th century furniture museum.
Kalmar in Sweden; truly, one of Europe's great medieval experiences. Wonderful location on a bluff above the Baltic Sea.
Frederiksborg Castle near Copenhagen is an amazing structure, but, as has been pointed out, it is more of a palace than a castle.
There is a small 15th century castle just North of Castelrotto, (Kastelruth) Italy, near the tiny village of Tagusens which is about 2 and a half miles beyond Castelrotto on a well paved, but narrow two lane road. The attraction here is that it takes a one hour down, hour and a half up again hike from the village to get to the castle and back. Consequently, it has never been disturbed and is totally authentic. There are tours in Austrian and Italian. There won't be many people (the hike limits the tourists). But, if you plan to be in this area, don't miss it.
In Ireland, I prefer the Rock of Cashel, even though it is a ruin. Same for Caernarfon in Wales; a ruin, but very worthwhile.
Carrickfergus Castle in County Antrim, Nortern Ireland will, again, meet your description in every way; 12th century, massive with all you have asked for.
In Belgium, Le Chateau de Bouillon near Namur is a fortress that will fit your description. Very impressive on it's rocky perch above the River Semois and it is the quintessential military stronghold.
Hope you find what you are lookig for. Have a great trip.

There is a 13th century castle in Meersburg, Germany that is excellent. Almost reminds me of "cave" times with skins hanging on the wall. Haven't seen it mentioned but I always remember it.

Another vote for Chateau Chillon on Lake Geneva! Just saw it last weekend and thought it was cool.

My all time favorite though is Dunottar by Stonehaven, Scotland. That's way far cool. :)

Cesky Krumlov is in the Czech Republic south from Prague. The most beautiful town in CR (incl. Prague)

Hi. By the way, if you do libraries, there are several great books of photographs of castles, and it would help your brother choose for himself.

As far as recommendations, I'll second the one in Meersburg on Bodensee (there's a wonderful collection of decorated knight's helmuts) and Chillon at Lac Leman. I've heard that Burg Eltz on the Mosel is outstanding, so you probably should include that.

Another 'did you know'--somewhere I have a book called "Castle Hotels of Europe". Try a search at Amazon. They might not be the most famous castles, but I'd imagine your brother would love to stay in a couple.

And don't forget to include a couple of walled towns. Rothenburg ob. T. in Germany, though hugely popular, has a Museum of Medieval Crime that is fascinating. Sounds like a dandy trip! J.