Complex sentence: This new castle was to be the new capital of the existing Owari Province and many of the materials used were sourced from the smaller Kiyosu Castle, including Kiyosu castle's "tenshu", which was located in the existing provincial capital of Kiyosu.
Split into simpler sentences: This new castle was to become the new capital of the existing Owari Province. The source for many of the building materials for the new castle was from the smaller Kiyosu Castle, including Kiyosu castle's "tenshu", which was located in the existing provincial capital of Kiyosu.