You can read the Imperial Library's walkthrough on Battlespire for info on the Battlespire, it might have some good information:
http://www.imperial-library.info/tsobs/part01.shtml
Also, Umbriel (in the novel The Infernal City) was a pocket realm. Here's how it was created:
*SPOILERS*
*SPOILERS*
*SPOILERS*
Page 279:
"Vile had made it impossible for Umbra to leave his realm, and after your escape, Sul, he tightened his walls further so that I couldn't leave either, even if I had the means. The only way to escape was to circumvent his restriction, to remain in his realm, at least in a way. I built my ingenium, I powered it with Umbra and the energies he had stolen form Vile. I turned our city, wrapped those circumvented walls around it. Twisted it like a sausage maker twists a casing to form a link, the way a child might an inflated pig's bladder to form a double ball. Twisted it until it broke loose, like a bubble."
"That was a long time ago," he went on. "We've drifted through many realms and places beyond even Oblivion. We cannot leave the city--Vile's circumvention still surrounds it. Nor would I want to leave it--I've come to love this place I built. To survive in those long spaces between the worlds, we had to become a little universe of our own, a self-sustaining cycle of life and death and rebirth, a continuum of matter and spirit--all powered, manipulated, mediated by my ingenium. We've moved beyond the inefficiency some call 'natural,' and in doing so approach perfection. Everything here is in a real sense a part of everything else, because all flows from the ingenium.
Long story short, Umbriel can break free of the pocket realm (which is really a pocket of Vile's realm) by using the White-Gold Tower to break free and become a part of the mortal plane.
Oh, and in case you're confused, Umbra is the soul that was in the sword Umbra, and the ingenium is a machine that runs on souls.