“Secluded Tranquility,” youjing, 幽静, is a possible translation of the plaque above the moon gate. The dictionary defines youjing as “quiet; secluded; isolated; peaceful.” (By coincidence, the word you, 幽, will also be a part of post 20.0.)
The word you, 幽, means “remote; hidden away; secluded; serene; peaceful.” The ideogram is an image of smoke rising like silk, 絲, from a fire, 火 . (The ideogram for fire was changed to 山.) The image of smoke rising from a mountain invokes being remote and secluded. The second word is jing, 靜. It means “still; calm; gentle.” The ideogram combines “blue, green, young,” 青 (picture of green plants growing near a well) with the glyph, 爭, which happens to mean “dispute; fight; contend” (picture of two hands grappling for a spear). To be tranquil means to stop fighting.
Walking into the Astor Chinese Garden Court did give me a feeling of tranquility. Based on the famous Master of the Nets Garden in Suzhou,* the court’s proportions, white walls, beautiful architecture, flawless workmanship, and gentle, still, light settled me profoundly.

I’ll never own such a garden court or the adjacent hall (16.0). But I can keep the ideal in mind or enjoy locations open to the public. Part of me would like nothing better than to be an old man in a courtyard. That won’t be possible, but the hope is there in me: the possibility of a peaceful space.
What is tranquility? It’s to stop fighting, to stop grabbing for the spear, to avoid stalemates. It is to support both the new plants and the well—both signs of life. If we give ourselves to secluded tranquility, we find healing, true meaning—and contentment.
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The Astor Chinese Garden Court. Ming dynasty (1368–1644). 17th century style. China. Taihu rocks, granite terrace, ceramic tile flooring, roof tiles, and door frames, various woods (nan wood columns, pine beams, gingko latticework), brass fittings. TheMet.
https://www.metmuseum.org/art/collection/search/78870
* https://en.wikipedia.org/wiki/Master_of_the_Nets_Garden