End of the 8th century a monastery existed here, owning the relics of Saint Kastulus, that had been "translated" to Moosburg from Rome. This monastery was converted into a collegiate, after the last Benedictian monks had moved to Weihenstephan Abbey in 1021. After the old abbey church had collapsed the erection of a new church started in 1170.

This was done under the guidance of Albert I. (aka Adalbert I.), the Bishop of Freising and so there are many parallels between these two large structures, built during his time. The Freising Cathedral was the first large brick building in Bavaria (after the Romans had left) - and St. Kastulus in Moosburg was the second.

The Gothic choir added to St. Kastulus later cannot be seen from this pov. The left tower is the tower of the former parish church St. John the Baptist.