Aqua-City was an aquatic city that served as the home of the Aquans on the planet Argo. It also served as the Aquan's capital city, as it housed an audience chamber for the Ruling Tribunal of the Aquans.
Upon discovering the Aqua-City in 2269, Spock found it "fascinating, and beautiful", while James T. Kirk noted the "fantastic architecture." Kirk also observed that "the building in the center; it seems to be important," while Spock added that they should enter the city "on the far side; there is much less activity there."
After a series of seaquakes returned the ruins of an ancient sunken city to the surface, allowing the Aquans to returned to life on the surface, Domar and the rest of the senior Aquans elected to remain there instead of re-settling the land masses on Argo, as they were not eager to become air-breathers again, unlike the younger generation. However, they assured Kirk that they would pass ordainments in order to not lose contact with each other. (TAS: "The Ambergris Element")