The Marches (an old term meaning "border country") is the term celestials most often use to refer to the Ethereal plane. It is a literal dreamworld, kept alive by sleeping humans and animals the world over. Its native inhabitants are referred to as "ethereal spirits" and can include minor background characters in a dream, famous legends, or even former pagan gods.
Few places in the Marches are truly fixed, but a few do stand out, even in this strange realm:
The Vale of Dreams is the most central portion of the Marches, where human dreamscapes (see below) tend to congregate. The Vale is claimed by both Blandine, Archangel of Dreams, and Beleth, Demon Princess of Nightmares; the unofficial border between the two runs down the center of the Vale, with pleasant dreams congregating on Blandine's side and unpleasant ones on Beleth's. Their two Towers dominate the horizon here, as does the Volcano of the Archangel Gabriel, whose Word includes the fire of inspiration that has touched many a dreamer.
The Far Marches are the forgotten corners of the dreamworld where the old pantheons hide and maintain their domains: Asgard, Olympus, the Happy Hunting Grounds and so on. Most are far weaker than they were in their heyday, thanks to the Purity Crusade led by the Archangel Uriel.
The Border Marches are the regions that separate the Vale from the Far Marches, carefully watched by Malakim to make sure nothing infiltrates. This is also the area where the Tsayadim are rumored to roam -- old Angels of Purity that never sought a new commander but instead continued their work of hunting down dangerous ethereals.
Since human dreams write most of the Marches, the centers of those dreams can be especially powerful. Each human moves around the Marches in a "dreamscape," a self-contained reality that is nearly opaque from the outside but could be anything on the inside: a forest, an alien planet, Yankee Stadium or something else altogether. Billions of these "bubbles" flow back and forth across the Vale constantly. A few celestials have learned the trick of entering a human dreamscape, but an alert human can expel intruders from their dreams -- often spending all their Essence to do so. Many Nightmare demons and ethereals will try to push humans to this point, hoping to catch some of the power that is expelled.
Humans who die in the Marches sometimes become dreamshades, tying their spirit to the dream world rather than flying to Heaven or Hell. Such a dreamshade can "move on" at any time simply by releasing its grip, though some are more aware of that than others.