Here is a description of the facade from the Cathédrale Notre Dame de Paris internet site.

- the square stands for created, limited space.
- the circle stands for the boundless, the perfect figure without beginning or end, the image of God. God’s world breaking into the created world, God becoming man. That is the mystery of the Incarnation. The heads of the Virgin and Jesus fall directly into the centre of the West rose window. Mary’s acceptance allowed God, as Jesus, to come into the mortal world, and Mary presents her Son to the city.

For more information here is a link to the site.
[url http://www.notredamedeparis.fr/The-west-facade[/url]