A milky-blue ocean of freezing liquid, more viscous than water, at nearly absolute zero (or the local equivalent). The air warms rapidly up from the ocean, to human-habitable fairly quickly (less than 60m), then more slowly to intolerably hot. The dark blue sky swarms with thousands of multicoloured, rapidly moving tiny suns, so that it never gets darker than a late-twilight level.
The peaks of a range of under-ocean mountains form the only survivable way through the shadow, in a linked, roughly linear chain of peaks formed of white, greenish-white and pale grey rocks. Beautiful crystalline plants in many colours grow roughly 30m up the lower slopes of the mountains; any higher and they melt.
A wide white road, now barely used, runs along the mountain ridge, through tunnels, over bridges and around cliff faces. Fortress Manasa is on one (the higher) edge of a gap in the mountain chain, overlooking a long, tall, multi-arched white bridge/viaduct over the gap.
The fortress looks like a huge, symmetrical black rose built on the final mountain peak, over one end of the bridge. There are several 'rings' of thick 'petals', containing rooms and the like, linked in a defensive array about a 'bud' in the centre which contains the larger rooms and halls. The 'rings' are linked by underground tunnels, and graceful enclosed bridges between them.
Up close, the black walls of the fortress seem carved of space itself - they appear to be clear black space, with stars gleaming in the depths. They reflect the images of living things, but do not reflect the sky or ground.
A roadway winds up from the road beneath the fortress to a gate in the wall between layers of 'petals', via a number of what are obviously kill-zones.
Although the walls appear uniform from outside, from inside they form windows and the like wherever needed, as well as defensive firing positions and the like.
In extremis, the whole fortress can fold up into a relatively compact, even more defensible 'bud' form.
As well as the normal rooms inside, the fortress also has many shadow fragments 'knitted' into it, providing all the facilities one might ever need.