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!
This was my first time ever visiting Oi-juku! It's said to be a post town where Emperor Meiji stayed, and you can really feel the remnants of that era. I was pleasantly surprised to learn that Ena City is promoting the Nakasendo road to this extent! The explanatory sign and the diagram of the square in front of the main inn were very easy to understand! What I found most amazing was that people still live in the main inn!