Zenkoji temple is definitely the most popular tourist destination in Nagano city and probably the one of the most famous temples in Japan. As like Muslims head to Mecca, praying at Zenkoji became a custom for Japanese once in their lifetime during Edo period.
The temple has a very long history, built in 644. The main building called Hondo, became the Japanese national treasue in 1953. Then, City of Nagano developed as the temple town, business related to Zenkoji. These cities which developed with major temples are called MONZENMACHI.
Zenkoji temple can be visited from Nagano station on foot but it takes around 20 minutes. Then, due to Nagano is connected with Tokyo by Hokuriku Shinkansen, it is possible to visit Zenkoji temple as a day trip. Or recommend to stop over when traveling from Tokyo to Kanazawa. If you are coming from Nagoya or Matsumoto, take limited express Shinano, which connects Nagoya and Nagano via Matsumoto.