I zigzagged from bottom to top. Some aspects of teh world were created from the bottom up, and others from the top down. The original germs of the world were two quotes, one from Keats's Ode to a Nightingale ("Whose voice hath oft charm'd magic casements, opening on the foam of perilous seas in fairylands forlorn.") and one from an old B&W movie called The Ghost and Mrs Muir ("Oh! I feel as if the house itself were welcoming me"). From those I deduced my way back to animism, a house with permanent portals in it, and a bunch of elves who have deserted their 'fairyland'. I added the high-level idea that the collective unconcious might be PROM rather than ROM, and that dreams might be shared, even become addictive. I filled those in with specifics, and then extrapolated my way back down ot local detail. Made a few calculated changes from fantasy cliché at the broad-brush level, and extrapolated down to the local scale again. Threw in a couple more local conditions, deduced my way back to general principles, and then extrapolated back to the bottom again. Rang the changes on some old tropes, deduced up, extrapolated down. Improvised some stuff for off-the-cuff adventures, worked it in, reconciled it with earlier posits, extrapolated a bunch more detail.