St. Ambrose began the construction of the Basilica Virginum, which was completed by his successor Simplicianus (Bishop of Milan from 397 to 400 or 401 AD.) Exterior walls are original from the late IV century.
See:
- Andre Grabar, L'Âge d'or de Justinien, de la mort de Théodose à l'Islam, Paris 1966
- http://www.medioevo.org/artemedievale/Pages/Lombardia/SanSimplicianoaMilano.html