I have a kobudo bud that is traveling Japan documenting old dojo. He is a good photographer so I hope he makes his research into an article I can post here.
The Sainekan 済寧館 is the dojo inside the Imperial Palace, Tokyo.
Established 1883, the current building was finished in 1933. Used near daily for the judo and kendo practice of the Imperial Guards Police. During the Occupation it was one of the only places where the police were allowed to practice budo nonstop. Postwar, when the north garden was opened to the public, a very tall hedge obscures the entrance used only by the Emperor, who still observes a special ceremony every ten years, and the Crown Prince, who attends his own special ceremonial demonstration every five years.
Most photos of the interior are shot from the right, facing the Emperor's box, and often don't show the judo tatami, which lie to the left.