The provided source material does not contain information regarding a city named Willa in South Sudan. The source identifies "Willa" primarily as a feminine given name associated with various historical figures, including queens of Provence and Tuscany, as well as several American and international actresses, authors, athletes, and activists.
Because the source material lacks any geographical data or descriptions concerning a city in South Sudan, no further details can be provided. The information is limited to the name's use as a personal identifier for notable individuals such as aviator Willa Brown and novelist Willa Cather.