Russ Nicholson's rough layout for the cover of The Court of Hidden Faces, of course. It was a good long time ago, so hard to say now whether this was done before or after Kevin Jenkins drew his pencil version. I used to design the covers myself, so it's possible that I sent that sketch to Russ along with the descriptions for the interior illustrations and he decided to have a go at it.
Publishers' art directors used to have a fixed sense of whether an artist could do maps, drawings or paintings. They might allow that somebody could do two of those, but never all three. Jamie and I, having worked with Russ on many projects, know how incredibly versatile he actually is, but even so it took us until the last two FL books to get the publisher to let him do the world maps.
One nice little detail you can see here: the mechanical buttresses on the palace that enabled it to walk. That was Russ's idea, and one we happily incorporated into Harkunan mythos. And it must have been a decade before Mortal Engines, too. And did Edgar Rice Burroughs get there even earlier, or was that a Disney retcon?