On December 11, 1930 (Shōwa 5), ownership of the castle was transferred from the Imperial Household Ministry to the City of Nagoya, thus abolishing its status as an imperial villa. In the same month, 24 structures on the castle grounds were designated as national treasures. On February 11, 1931 (Shōwa 6), the grounds were opened to the general public. The next decade saw conservation and archaeological activities and the castle was scientifically documented. In May 1932 (Shōwa 7), a field survey and measurement of the castle were carried out. In July of that year, the old "Kayanoki" (Japanese nutmeg) tree in the Nishinomaru was designated as a national monument. In December the castle was designated a historical site. In 1936 (Shōwa 11), the Sarumen Tea House (猿面) in the Ninomaru was designated as a national treasure. In June 1942 (Shōwa 17), some of the Honmaru Palace paintings were designated as national treasures. Most of the sliding doors and paintings were put into storage as World War II threatened the Japanese mainland.