My first thoughts on this was the fort doesn't have enough windows. In the book the travelers were able to see a myriad of windows shining out light. Furthermore, the house of Harfang shouldn't look like a castle but a gigantic mansion. The giants of Harfang feared no attack and therefore didn't shut themselves up in fortifications but a comfortable abode. Harfang should show the sophistication of the giants that live there. It should therefore be decorated to some degree. However, it should still have a touch to crudeness to it, making a distinct difference between the ancient giants and themselves. The ancient giants were quite simply geniuses therefore their architecture could compete with the best masons of all time! The ruins should show this fact.

Well i would imagine they would mostly film such scenes using miniatures, CGI and indoor sets as they did for Miraz's castle in Prince Caspian, but as for a general guide for the look of the castle, i would probably go for something of an Eastern European sort of style castle, suitable for the cold weather environment of Ettinsmoor/The Wild Waste Lands. Not as blocky and fortressy as the one you suggested. Something more ornate, possibly a Bavarian castle, but i wouldnt want it to end up being too Harry Potter like though. Perhaps this one from the Czech Republic.

I think the place in the BBC version of SC was perfect! If only they hadn't used it. I suppose they could use it again, but that wouldn't be right.

Sorry I can't find a picture anywhere

I agree with you, EelStew. (BTW, love your username!!!!)
I did like the set in BBC SC.

I always pictured it like a huge castle that was kinda long:

I think the place in the BBC version of SC was perfect! If only they hadn't used it. I suppose they could use it again, but that wouldn't be right.

I have some photos of it but my scanner doesn't work.
It's called Haddon Hall, and it's in Derbyshire.
If you saw the recent Jane Eyre miniseries (in which Georgie played young Jane), you will have seen it as the house of Mr Rochester.

Wow, I love that building icarus posted a picture of: the one from the Czech Republic. That's almost exactly how I always imagined Harfang. Of course, I doubt they'll actually film a real castle for Harfang; they'll probably use miniatures, CGI, and custom built sets.