The original Iwakuni-jō was built by Hiroie, the first of the Kikkawa lords, between 1603 and 1608. Just seven years later, the Tokugawa shogunate passed a law limiting the number of castles daimyō were allowed to build, and the castle at Iwakuni was demolished. It was rebuilt not far from its original setting in 1960. There is nothing much of interest inside, but there are good views from the hilltop setting.
You can get to the castle by cable car (one-way/return ¥320/550 or combination ticket including castle and bridge return ¥940, every 20 minutes), or you can walk up to the castle among the greenery and birdlife on a pathway that leads up from the west side of the park. The walk takes about 45 minutes.