I didn't read it, though, because I discovered I had a more complete version of the story in the first volume of the Nostalgia Ventures two-fer series. The Nostalgia Ventures edition features all the original art from the October 1938 issue of the pulp, so I read that instead.
According to Will Murray's introduction, Street & Smith gave the pulp story extra promotion. Readers had been teased with hints about Doc's arctic hideaway for five years, and their first peek inside was treated as a special event. Lester Dent apparently agreed, and gave the story a special villain, an evil genius and master hypnotist who calls himself John Sunlight.
As a story, Fortress of Solitude is pretty standard stuff (weird menace, gang of thugs, Doc and/or his aides getting captured), and we get only two paragraphs describing the interior of Doc's dome. But John Sunlight is undeniably special - if only for the fact that he escapes Doc's clutches, taking with him a score of deadly devices, and returns two months later to battle Doc again in The Devil Genghis. Death's Dark Domain takes place in between those two adventures, as Doc and the gang face the consequences of Sunlight's theft.
Doc's Fortress as seen in the pulp.
That distant figure whacking it with an ax is John Sunlight.
I'm reading Death's Dark Domain now, and will have a report on it soon!
Links to more Forgotten Books at pattinase.