The person who first unified Japan was Oda Nobunaga.Later, the subordinate Toyotomi Hideyoshi betrayed and killed Oda Nobunaga. Then, Toyotomi Hideyoshi was killed by Tokugawa Ieyasu, who was close with Oda Nobunaga.This story is a very famous tale that all Japanese people know.The personalities of these three individuals are expressed in the haiku that was written about the bird called the hototogisu.
・Oda Nobunaga　→　If the bird does not sing, then kill it.
・Toyotomi Hideyoshi 　→　If the bird does not sing, I will make it sing.
・Tokugawa IeyasuIf 　→　it doesn’t sing, I will wait until that bird sings.

There used to be a temple where Osaka Castle currently stands, but Oda Nobunaga, who recognized Osaka’s proximity to Kyoto, Nara, and Sakai, as well as the sea, seized the temple to build a castle symbolizing the unification of Japan. However, Nobunaga’s dream was unfulfilled and he was killed by his subordinate, Toyotomi Hideyoshi.
Then Toyotomi Hideyoshi built a magnificent Osaka Castle on the ruins of the temple and boasted about it to Tokugawa Ieyasu. However, during this time, he revealed to Tokugawa Ieyasu how to attack Osaka Castle. Some time later, Tokugawa Ieyasu attacked Osaka Castle as advised and captured it, subsequently destroying it. The current Osaka Castle is the splendid castle that Tokugawa Ieyasu built anew.

These three unified Japan and have influenced the entire country. Therefore, Osaka Castle is the most famous castle in Japan among many.