Kain's Mountain Sanctuary also known as Kain's Mountain Retreat was one of the final levels originally planned for Soul Reaver that was removed due to deadlines.
In the original storyline, Raziel would have been able to reach Kain 's retreat by using an updraft from Turel's smokestack to glide into the mountains and up to the secluded retreat. Here, armed with the Ariel Reaver, he would have had his final battle with Kain, finally killing him and taking his Soul into the Reaver to form the Amplified Reaver(also known as the Kain Reaver.)He would have been able to then use the height of the mountain to glide to the Pinnacle of the Silenced Cathedral.
The exact placing of Kain's Mountain Retreat within Nosgoth is unsure, some sources place the Retreat to the East of the Abyss, between the (also removed) the Smokestack and Pinnacle of the Silenced Cathedral, (although unlike these locations Kain's Sanctuary is not seen in the opening video); others consider the Sanctuary of the Clans and Sarafan Tomb to be closer to Kain's true interests and place his territory between the Drowned Abbey and the Sunlight Glyph Altar, so his territory covers these locations.
The only known footage of Kain's Mountain Retreat comes from the visions in the Chronoplast where Kain and Raziel face each other, curiously Raziel is armed only with the Material Reaver here, not the Ariel Reaver - this scene would eventually become part of the Legacy of Kain: Defiance storyline.
Concept art from Soul Reaver depicts an apparently similar mountain-top structure, however this has been confirmed to be an early version of the Sanctuary of the Clans.