It is the palace where the generations of Emperors and Empresses of Japan have lived starting in the Meiji era (1868-1912) to this day. You can visit the palace site as long as you obtain prior permission.
The palace used to be the castle (the Edo Castle) of the Tokugawa shogunate, but it became the palace for the Imperial family in 1868. It had been called kyujo (the Imperial castle) since 1888, but in 1948, it was renamed to kokyo, the Imperial Palace. Within the Imperial Palace premises, there are 1) gosho, where their Majesties, the emperor and empress, reside; 2) kyuden (the Imperial Hall); and 3) other buildings for the Imperial Household Agency government offices. In one area of the premises is Kokyo Higashi Gyoen (the East Gardens of the Imperial Palace), where the Tokagakudo Concert Hall is. Also, a general visit tour that allows visitors to see the inside of the Imperial Palace site is available. Although you are not allowed to enter the buildings, but you can still look at the palace premises or the government office buildings of the Imperial Household Agency. Other than the buildings, there are many places where you can enjoy nature during the summer. These places within the Imperial Palace site include the Lotus Moat, where you can see large lotus flowers, and Yamashita Street, which is quiet and full of greenery. The general visit tour takes about an hour and 15 minutes, and prior permission is required. For the access to Kikyo-mon (the Kikyo gate), the meeting place for the general visit tour is about a 10-minute walk from Exit No. 6 of the Nijubashi-mae Station on the Tokyo Metro Chiyoda Line and the D2 Exit of the Otemachi Station on the Metropolitan Subway Mita Line.