The city is the symbolic creation of a society in which women are valued. Reason is personified as the architect of the city.
What Pisan is after is creating a perception of women as important to society. She does not want to replace patriarchy; rather, she wishes for an honest appreciation of the contributions of women in the world.
To do so, a new city has to be built. Women must excavate their worth from the mire of male misconceptions (both deliberate and sometime subconcious) about what women can and cannot do. They must stand up for who they are rather than passively accepting how they are perceived.
Reason instructs Pisan that this recovery work must be done by women themselves...they have to prove to men through the power of their pens that they are logical thinking human beings, not ruled, as men charge, by passion.
Once the foundation of logic has been layed, the "bricks" that go into the walls are the examples of venerable women, both past and present. (You can think of this as a thesis statement with supporting evidence.)
To make the walls strong, however, women cannot just rely on the virtuous work of those who have come before them. The bricks must be continually added to the walls and, again, it is women who bear the responsibility for maintaining the defense. They do this by being well-read, well-traveled, and above reproach morally.