I should mention that there had been many floating castles before the Interregnum. I guess the spell isn’t all that difficult, if you care to put enough work into it in the first place. The reason that they are currently out of vogue is the Interregnum itself. One day, over four hundred years ago now, sorcery stopped working … just like that. If you look around in the right places in the countryside you will still find broken husks and shattered remnants of what were once floating castles.

Steven Brust (b. 1955) American writer, systems programmer
Jhereg, ch. 7 (1983)