Purpleopolis was the most important hippopotamian city in the classical mythology. Located on the continent of Hippodom, it was built with the help of dwarves right after the races awakened. Apparently, it was circular and fucking fucking huge. There's a disturbingly detailed description of it in the myths:
Purpleopolis was situated on the western side of the Purple Mists, wedged in between two mountains and controlling the flow of the Crystal River which issued from the Purple Mists.
The city was built in a circular fasion, consisting of 3 rings. In the innermost ring was the Citedel of the Golden Hippo and its many river-gardens. The second ring had 3 Temples inside it, and by each of them was a great tower, and in between them was all other buildings that were not residential. Also in the second ring was the great Crystal Pool with the waterfall from the Purple Mists feeding it. The outermost ring consisted entirely of housing, except for the 5 recreational pools. Altogether, the city was 27 miles in diameter. The first ring was 7 miles across, in between the walls of the first and the second rings was 4 and a half miles, and in between the second and the third walls was 5 and a half miles.The city itself was built by the Dwarfs, though it was concieved of by the FPH's, and the Hippos also created the swamps that decorated the city, and the river-streets that ran through and around it. With the two races combined arts, it became the most beautiful city in the whole of the World.—Gagylpus, The Beginning Days