It's nice, you can find remnants of the Nakasendō here for sure. They explain some in English, too.
Never been
This main fortification site can only be viewed from the outside; the interior is not open to the public.
However, the 1/4 scale Meiji Emperor's Imperial Residence in Oi and the Nakasendo Hishiya Museum are both open and open for visits, which is great!
Seeing it in person gave me a sense of self-satisfaction. lol
From the side, it looked quite rough.
I kind of understood why you could only see the exterior.