The Seven Mounting Heavens of Celestia are one of the Outer Planes, and home to a variety of Archons, and creatures seeking redemption/ enjoying their afterlife.
The first layer of the plane appears to be eternal night time, a beautiful unobscured starlit sky over a silvery sea that extends to infinity where first-layer portals appear to drop people out.
A central landmass near where the party emerged on their visit holds an incomprehensibly large mountain that extends out of sight up into… well, into the heavens. At the base of this is a white-stone citadel for arriving individuals to relax in, before moving up the mountain.
The citadel was constructed uncounted aeons ago by someone seeking redemption.
Known entry portals:
Wyvernhill → 1st layer, ocean
Casswyn’s shack → 2nd layer
Known exit portals
2nd layer → Casswyn’s shack