The entrance fee for Hiroshima Castle’s main tower is ¥370 for adults[1][5][7]. Seniors (65 and over) and high school students pay ¥180, while junior high school students and younger can enter for free[5]. Access to the castle grounds, including the Ninomaru compound, is free of charge[1][7][8].
Please note that the main keep requires a paid ticket, but exploring most of the outer grounds and structures does not[1][7][8].