Why go
The castle district makes more sense when seen as the site of a planned political center rather than an isolated monument. Terumoto moved his base from inland Kōriyama Castle to the delta, but lost Hiroshima after the Battle of Sekigahara in 1600. That change of rulers links the city's founding to the start of Tokugawa rule.
The full story
Mōri Terumoto chose the Ōta River delta for a new base in 1589, leaving Kōriyama Castle in inland Aki Province. Hiroshima Castle rose quickly, and Terumoto moved into it in 1593. The setting gave the castle town access to river routes and the Seto Inland Sea. Its early political life was short under the Mōri. Terumoto backed the losing side at Sekigahara in 1600, and Tokugawa Ieyasu transferred control of Aki Province to Fukushima Masanori. Those events are the useful thread to carry into the castle area: Hiroshima was established as a new regional center just before a major shift in Japanese rule. The present castle landscape is later than Terumoto's first buildings, but its location still marks the choice that set the city's shape.