Korakuen is one of the Big Three Famous Japanese Gardens (Nihon Sanmeien 日本三名園). The other two are Kairakuen in Mito, Ibaraki Prefecture famous for ume plum blossoms and Kenrokuen in Kanazawa, Ishikawa Prefecture famous for large pine trees. No one knows for sure who selected these three as the Big Three nor the criteria for their selection. No one knows who first coined the term either, but the first known appearance of “Nihon Sanmeien” was in a Japanese book published in 1899.
One theory is that these three gardens were selected to represent the traditional artistic triad of Snow, Moon, and Flowers (雪月花) commonly found in Japanese art and expression. Kenrokuen is snow, Korakuen is moon (autumn), and Kairakuen is flowers (spring).
All three gardens were also created by their respective local samurai lord during the Edo Period (17th c. to 19th c.). Korakuen was originally completed in 1700 after 14 years of construction. All three gardens are classified as a “circular-strolling Japanese garden” (kaiyu-shiki teien 回遊式庭園). A common and classic Japanese garden design where you simply walk around the garden, usually around a central pond. The strolling path is not a perfect circle, but you can start and finish at the same point.
In the case of Korakuen, the strolling path is pleasantly lined with Japanese pines, flowering shrubs like azalea, and green lawns. There are also walking paths crisscrossing the garden (lower left photo). Lots of greenery. Even the pond looks green. It’s an expansive, open-air garden with minimal tree cover.
During World War II, Korakuen and Okayama Castle were damaged by bombing raids in June 1945. By 1967, all the garden’s structures such as the Noh stage and guesthouse were reconstructed. Korakuen is owned by Okayama Prefecture.
Korakuen was also designed to integrate the surrounding backdrop such as Okayama Castle across the river from the garden (upper left photo). Nicknamed “Crow Castle” because of its black exterior, Okayama Castle is an authentic-looking, modern reconstruction. Inside is a modern building with some exhibits and bird’s-eye views of Korakuen. Very few Japanese gardens have a castle in the background.
Korakuen is actually built on an island in a river. There’s a pedestrian bridge from the garden that crosses the river to the castle. Two of Okayama Prefecture’s most famous tourist attractions are conveniently right next to each other. If you visit one, you must also visit the other. (Not to be confused with Koishikawa Korakuen Garden in Tokyo.) 
From JR Okayama Station, it’s a short bus ride from bus stop 1. Get off at Korakuen-mae.
Official: https://okayama-korakuen.jp/section/english/