The city of Civitas Sykofantia sits on the shore of Lake Sykofantia, a high-elevation salt lake the size of Lake Superior in the US. The lake is located on the Great Pagomenos Plateau, a huge escarpment 9,500 feet above sea level, extending south from the Pagomenos Mountain Range. Due to elevation the city has a frigid subarctic climate.
The city has great economic wealth. It exports huge amounts of lumber, salt, osmiridium (a fictional metal), and caviar from a local sturgeon species. They export goods using flying sailboats called startreaders (use the link for context) that take off and land on the lake.
The city has a population of at least 100k. How would a civilization with a late medieval/early renaissance tech level feed and hydrate such a city?