Rather than being built atop a hill or Mountain, Hiroshima castle, aka Carp Castle, is located right in the centre of the city, with stunning views from its top floor.
Having been spared the wrath of the Meiji-demolition which destroyed many other castles, it was later destroyed by the atomic bomb, and restored in 1958 to its current form. The inside is filled with the history of the city, castle and samurai culture across five floors with English information provided, but fading as you get higher up, oddly.
The castle grounds have a shrine with entwined fish for hopes of finding your perfect lover, as well as numerous ruins and more recently rebuilt structures from the Ninomaru (second circle of defence). The gardens are great for a stroll and are free, although entry to the main castle keep is ¥370.