The original Osaka Castle was built by Toyotomi Hideyoshi between 1583 and 1598. It has been destroyed and rebuilt several times since that time. The current concrete reconstruction is based on the version of the castle that was built by the Tokugawa Shogunate, and which was quite different to the Toyotomi version of the castle. Read more about Osaka jo here and here.
Read more about the day I went to Osaka jo here.