There is a brick-vault roofed bathroom building, commonly known as the Cistern Hall (Wengtang), opposite the Grand Baoen Temple Heritage and Scenic Area near the Zhonghua Gate in Nanjing. This article analyzes its shape and characteristics by means of a 3D scan that surveys and maps out 2019 data on the site. On this basis, this article suggests that the bathroom may have been built as an annex to the Grand Baoen Temple in the early Ming dynasty. This kind of bathroom design originated in West Asia, entered China during the Yuan dynasty (1271-1368) and became popular in the Jiangnan region during the Ming (1368-1644) and Qing (1644-1911) dynasties. In a lively exchange of architectural ideas, it bore witness to rich, in-depth architectural cultural exchanges between China, Central Asia, and West Asia in the Yuan and Ming dynasties.