Azuchi Castle in the context of "Ōmi Province"

Oda Nobunaga (織田 信長; [o.da (|) no.bɯ(ꜜ).na.ɡa, -na.ŋa] ; 23 June 1534 – 21 June 1582) was a Japanese daimyō and one of the leading figures of the Sengoku and Azuchi-Momoyama periods. He was the Tenka-bito (天下人; lit. 'person under heaven') and regarded as the first "Great Unifier" of Japan. He is sometimes referred as the "Demon Daimyō" and "Demon King of the Sixth Heaven".

Nobunaga was an influential figure in Japanese history and is regarded as one of the three great unifiers of Japan, along with his retainers, Toyotomi Hideyoshi and Tokugawa Ieyasu. Nobunaga paved the way for the successful reigns of Hideyoshi and Ieyasu by consolidating power, as head of the very powerful Oda clan, through a series of wars against other daimyō beginning in the 1560s. The period when Nobunaga and Hideyoshi were in power is called the Azuchi–Momoyama period. The name "Azuchi–Momoyama" comes from the fact that Nobunaga's castle, Azuchi Castle, was located in Azuchi, Shiga; while Fushimi Castle, where Hideyoshi lived after his retirement, was located in Momoyama.

The Japanese Sengoku period from the mid-15th to early 17th century was a time of nearly continual military conflict. Powerful military lords known as daimyōs, such as Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu, struggled to unify Japan. During the Sengoku period, because of constant warfare, many fortifications and castles were built. Archetypal Japanese castle construction is a product of the Momoyama period and early Edo period.

A new era of castle construction began when the daimyo Nobunaga built Azuchi Castle from 1576 to 1579. Earlier fortifications of the Kamakura and Muromachi periods were crude large-scale structures; Azuchi, however, with rich ornamentation and a keep rising seven stories high, became the prototype for castle construction of the period. The style of Azuchi Castle marked a shift in the function of the castles from a place that was merely a fortress and military garrison to a political, cultural and economic center. The newer style castles functioned as home to the daimyōs, his family, and his most loyal retainers. Because of the expense of building such a lavish structure, castles in the style of Azuchi, functioned also to highlight the power and prestige of the daimyōs. These new castles were built of wood and plaster on a stone foundation. Generally the main keep or tenshu was positioned at the highest point, surrounded by a series of interlocking baileys with walls, small towers and pathways. Residential buildings were located in one of the outer circles. The daimyō conducted his business in the citadel.