Why did Oda Nobunaga build the castle in a delicate place called Azuchi?

It's not subtle, it's in a commercial, military and politically important position.

1) Connecting Kyoto and Gifu

At that time, the Oda family's economic and military bases were centered on Owari, Mino, and Yamashiro (Kyoto), Gifu in the east, and Kyoto in the west. Gifu Castle was an important military base for the Oda family and an important military base for the Oda family's direct taxation of Mino and Owari.

Regardless of Kyoto's political importance, it was the long-term capital of the Ashikaga government, and kyoto, which had seized Nobunaga's political importance, was an important city to maintain the legitimacy of the new government.

However, the land route from Gifu to Kyoto is 120 kilometers, and even though I was at the foot of the army, it was a little far to come and go. Therefore, Azuchi, which is 1 days away from Kyoto and 2 days from Gifu, can be said to be the midpoint.

2) The economic network of the area, with rivers and lakes as the main sea route treaty ports

Nobunaga's war seems to envisage the seizure of economic benefits linking the ports of Osaka, Nagoya, Kyoto, and Kanazawa with Lake Biwa and the river. After capturing Kyoto and its surroundings with Tenkafu, they attacked Asai and Asakura, captured Ise, confronted Honganji Temple, and attacked areas that were crucial to Kyoto's economic development.

Lake Biwa is the center of Kinki's economic network. The most trusted of the Mingming Mitsuhide Sakamoto Castle is located on the west bank of Lake Biwa, and the east bank is solidified by the direct domain of the Oda family (to the north is the hokuriku of the veteran Shibata Katsuya). It seems that this plan is to distribute the above goods through Lake Biwa to all of, and establish a center of interest centered on the economy of Lake Biwa.

From the castle tower in Azuchi Castle, you can see the entire Lake Biwa on a sunny day, as if you can see everything in Kyoto and Sakamoto Castle.