Vitruvius Quote

After insuring on these principles the healthfulness of the future city... the next thing to do is to lay the foundations for the towers and walls. Dig down to solid bottom, if it can be found, and lay them therein, going as deep as the magnitude of the proposed work seems to require. They should be much thicker than the part of the walls that will appear above ground and their structure should be as solid as it can possibly be laid.

Chapter V "The City Walls" Sec. 1 - De architectura (The Ten Books On Architecture) (~ 15BC) - Book I