In the original release of D2:ED there were generators and anti-dragon zones in the flying fortresses, with a cutscene showing an overview when you entered. In DKS all but one the the generators and anti-dragon shields were removed (in Stone's fortress, where you encounter Xanlosch), and the fortresses themselves redesigned a bit.
The flying fortresses increase in difficulty as you proceed into the fjords, so from easiest to hardest: Stone, Raze, Broken Valley, Keara and then Kali. If you do the harder ones first you might not be getting much experience when you do the easy ones (then again you would get extra experience doing the hard ones at a lower level).
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