From a sociologic profile, the structure of the metropolis, in its subsequent levels (quarters or villages, townships, federation of cities) is analysed from an educational  point of view, from school to  University level , to which the city-nucleus of the Navigli is dedicated. From a religious or, even better , an esoteric point of view, to which the book's title tacitly refers, the nucleus is the very heart in which Man discovers his Divine essence. The void is the place of the essence, the seat of the Soul. A new philosophy about the city can only be based on an inner truth; and this is why the ideal city can only be in man's image. Finally, the project may be interpreted as the city's development to a harmonic degree, compatible with the safeguarding of civic and natural values. It is not a blind and unlimited expansion, as often happens for many of today's features. To conceive the city-walls the other way round, as occurs in this project, is perhaps to make a decisive step towards a new civilisation. Once the city-walls were necessary to protect the city against the outer, hostile world, while they are now called upon to assume the inverse function: that of protecting the inner place - that is to say what is most dear to us - from the pressure of the city-outside-world. It may even be an anticipation of what awaits the Earth tomorrow: to be a heart-nucleus dedicated to safeguarding the memories and those few natural elements which may have survived and, all around in outer space, a crown of satellite cities in orbit ...