People frequently ask why the donjons of Japanese castles are empty.
People may imagin that buildings at the castle are filled with items such as armours, arms, furnitures etc. But unfortunately they are not. In fact, most of the buildings at the castle in Japan are empty.
The reason is as follows.
1. The remaining buildings in the casle of Japan are mostly donjons or towers. These were the facilities to protect in case of a battle and not a place to live. And also, it was built to show the dignity of the feudal lord to the citizens or oppnent samurai clans. Therefore, it was not meant to hose items inside.
2. There were buildings used as office, or residence, but most of them were lost since feudal system was abolished in mid 19th century by Meiji Restauration. And there often found are the stonewlls that served as a base to these buildings.
In some castles such as Nijo-jyo castle in Kyoto, or a castle in Nagoya, Hikone, you can see some of these building where you can see how it used to be the inside of the castle when it was in use during Edo period. Or, at Tamozawa Imperial Vila in Nikko, there is a beautiful building restored and moved from an ex-daimyo residence in Tokyo. You can see the authentic Japanese castle building by walking tatami floors there.
Also, it is fun to imagine the life of samurais ruling the territory even from the remaining ruins, because it will give you more imagination and inspiration.