I really don't feel I'm appropriating
Otherland into fantasy because of its feel. It kind of bugs me when people do appropriate things for their own agenda, I wasn't trying to do that with
Otherland, just to convice Archie to read it. However, there are many elements that are more recognizable as fantasy than sf. Here are a few things, in my mind, that put
Otherland more along the
fantasy root of speculative fiction
-The journey/quest aspect of the novel.
True some SF novels do have the theme of quest or journey, but for the most part, journey/quests are more a characteristic of fantasy, at least when compared against the sf I've read. For the most part Paul Jonas' story is a story based on a quest [/QUEST]
Then you're not reading enough sf.
Most sf involves a journey/quest plot of some sort. For that matter, the journey-quest idea is common throughout the lands of fiction. That the main character is trying to rescue her brother from those who've captured his mind does not make the work fantasy. It makes it a sf thriller actually.
-the big bad enemy-Felix Jonular
Whenever there is a protagonist, in any genre there is usually an antagonist. But again, IMHO, the "big bad enemy" is one of the stronger qualities of Epic Fantasy.
There is more than one enemy in Otherland. The enemy set-up, in fact, is far more characteristic of sf novels, with its corporate politics, than fantasy. As for big bad enemy, again, thrillers -- very common device. Does not make it fantasy.
-The odd, fantastical worlds in Otherland
from the world where Thargor/Orlando hangs out, to the almost fairy-tale kitchen, to the use of the African myths/vr worlds and !Xabbu's creation myths, there, in my mind, is a decidedly fantasy feel to it rather than sf
-The Great Temple/Tower etc
See Mountain of Black Glass both the book and the actual Mountain as the Dark Tower/Temple/Fortress etc.
The fantastical worlds of Otherland are not actual fantasy worlds. They are generated by a computer. They are no more magical than the Myst game in your computer right now. Having fantasy games in a VR network, or software programs that don't know they're software programs, or people who are trapped in the network and unaware that they aren't really in a fantasy world, or AI's that go rabid -- none of these turn the work into fantasy. They are more reflective of high tech and cyberpunk sf.
But really, we are just cutting hairs right now aren't we?
Yes, and Tad Williams will tell you he doesn't care what you call it as long as you read it because authors hate the labels anyway. But he did intend it as sf and the plot is sf. There are no fantastic elements -- no magical, supernatural, unexplained phenomena.
The reason I split hairs about it isn't that I'm obsessed with market categories. It's that I don't get why people ignore the entire plot of a book, and quite often much of the thematic and character development as well, because they don't want the story to be what it is. Here the author has gone to all this work to set up a complicated sf plot and because there's a virtual ogres and some virtual swords, it must be fantasy? I do think fantasy fans would mostly enjoy the Otherland series. But I do not agree with the trend to label any book that is enjoyed as fantasy, because, what, fantasy is the only acceptable form of story? Anything that employs anything remotely fantasy-like becomes a fantastical story? It doesn't make sense. But that's my bug-a-boo.
What's a Shadowmarch?
As for Otherlands series, can not say anything yet! That series is in my to-read pile. I will let you know what I think sometime down the road! 
