Ō-yagura was built in the mid-17th century. With the size of about 4 m x 6 m (approx. 13 x 19 ft), this three-storied watchtower was constructed by building up piles of stones. This was the biggest watchtower in Naegi Castle. The 1st floor enclosed by stone walls on three sides was used as a storage. There were loopholes on the walls on the 2nd and the 3rd floor for the purpose of defending the north side of the castle.