Tokyo Imperial Palace (皇居 Kōkyo; literally, "Imperial Residence"?) is the main residence of the Emperor of Japan. It is a large park-like area located in the Chiyoda area of Tokyo close to Tokyo Station and contains several buildings including the main palace (Kyūden (宮殿?), the private residences of the imperial family, an archive, museum and administrative offices.
The Imperial Palace East Garden (Kokyo Higashi-gyoen) is the only quarter of the palace proper that is open to the public. The main entrance is through Ote-mon, a 10-minute walk north of Niju-bashi. This was once the principal gate of Edo-jo; the garden lies at what was once the heart of the old castle. You'll be given a numbered token to turn in when you depart. The store inside the garden sells a good map for 150 yen.