Cherished by the locals, Mt. Otsu is a rather short mountain with an elevation reaching 256 m. It is known as “Nankan Fuji” due to its outward resemblance to Mt. Fuji. The Tsuzuradake Castle Ruins, which can be found on the peak, are said to be the remains of the mountain castle built in 1396 by OTSUYAMA Sukemoto, the first in the lineage of the Otsuyama family. This castle constantly came under attack and defended itself repeatedly over a 200-year timespan until KATO Kiyomasa constructed a new castle, Nankan Castle, also known as Nankan New Castle, in Takanohara, near the present-day Nankan Ochaya Remains, in 1600. Even today, traces of the structure, including the stonework and dry moats, continue to exist, conjuring up a once steadfast fortress.
There are two trails hikers can take to reach the summit: one route on the west that goes up from Otsuyama Aso Shrine to the top of the mountain, and another that starts from Otsuyama Park. Along the Otsuyama Park trail, four wooden bridges, each bearing the name of an Otsuyama family member, all of whom were once the lord of Otsuyama castle, straddle the castle’s dry moats: Sukemoto-bashi, or “Sukemoto Bridge”, Iekado-bashi, or “Iekado Bridge”, Shinzo-bashi, or “Shinzo Bridge”, and Akimoto-bashi, or “Akimoto Bridge”. The peak of Mt. Otsu overlooks the Kyushu Expressway directly below and provides an uninterrupted view of the town. And far off in the distance, one can see Unzen in Nagasaki Prefecture, just beyond the Ariake Sea.