Here it was raining... anyway here is the castle. The Osaka Castle is the best known sight in Osaka, it is located in a park in the central Chuo-ku area.
I suggest to visit the page on Wikipedia to have a nice overview of the Osaka Castle history. Anyway in a very short, the castle construction originally started in 1583 by Toyotomi Hideoshi. The basic plan was modeled after Azuchi Castle, the headquarters of Oda Nobunaga, but surpassed it in every way: the plan featured a five-story main tower, with three extra stories underground, and gold leaf on the sides of the tower to impress visitors.
After construciton was ultimated in 1598, the castle was the site of many big and importand battles, and was partially or totally destroyed many times (last one was during a bombing in 1945) and fully reconstructed (last time was only in 1997).