Iiyama castle was originally built by Japanese daimyo Uesugi Kenshin as a stronghold for his invasion of the Shinano (modern day Nagano) region. In the 12 years of warfare between Kenshin and rival daimyo Takeda Shingen, it's said that Iiyama castle was never captured. While the castle itself is gone, the entry gates, embankment, and stone walls are preserved in their original form.
Nowadays, the castle ruins park is one of the best places in the city to see gorgeous cherry blossoms, and hosts Iiyama's annual cherry blossom festival. Visit from late April to early May to see the flowers in full bloom!