Kaijo Park is located on the site of Yamagata Castle, in Yamagata. Recently, strong efforts have been made to complete renovation of the castle. On the 100th anniversary of the founding of Yamagata City, the Ote gateway to the castle was renovated and is being exhibited twice a year.

In 1356, Shiba Kaneyori built defensive stockades, which became the foundations of Yamagata Castle. From 1592, Mogami Yoshiaki, his descendant, remodeled it over 13 years and completed the present castle's form. After he started ruling his territory of 570,000 goku (a unit of land that can produce enough rice for one person per year), 12 custodians took over from him.

The remaining stone walls and moats give an indication of the original castle. Within the castle grounds is a structure called Saisei-kan that shows a Western style different to the other buildings.

Now, the park is famous as a place to view cherry blossoms and enjoy hanami parties in spring.