What is the minimum population that is a large and dense enough gathering of people to provide the opportunity for continued training in specializations? How many times was fire discovered, or the wheel, or the beginning principles of math, only to be lost when the individual with the spark of insight died? Civilization exists to protect voluntary safe interaction among individuals. Taking a long term view, in a low energy paradigm, the city needs to be an ecosystem where its living and non-living aspects have multiple and interwoven sources of feedback. PURPOSE MINIMUM SIZE HUMAN SCALE RETROFIT