Hidden Abyss Valley is different from Ascension Immortal Mansion. The Ascension Immortal Mansion is built atop an Immortal Mountain, whereas Hidden Abyss Valley is built on the shores of East Lake. It may be called a valley, but it actually resembles more of a manor. The reason it's named Hidden Abyss Valley is due to the phrase "heart as spacious as a valley."
What sets Hidden Abyss Valley apart from other Immortal Cultivation Sects is its construction by the water, not isolated from human settlements. According to the founding ancestor of Hidden Abyss Valley, the mountain scenery is always the same, whereas East Lake offers beautiful vistas: misty spring rains, blooming lotus flowers in summer, wistful water waves in autumn, and snow-covered winter landscapes.
Thus, Hidden Abyss Valley is located on the shores of East Lake.