Imperial Palace (皇居）

Formerly known as Edo Castle, this was the former residence of the Tokugawa Bakufu, and later became the residence of Emperor Meiji, and remains the Imperial Residence today. It is known as Kokyo, which literally translates into Imperial Residence. It is located in Chiyoda, the center of Tokyo. From a historical perspective, the location of this castle makes sense as it would have been the focus of the city Edo (now called Tokyo).
The wide grounds are, for the most part, not open to the general public, except on New Year's and on the Emperor's birthday, when the Imperial family makes a public appearance on the balcony of the Imperial Palace. However, the Outer Gardens and Kitanomaru Park are open to the public, and the Eastern Gardens are open during the day, except for Mondays and Thursdays.
Foreigners can join free guided tours of the Imperial Palace grounds, though bookings must be made in advance. These tour groups, being high in demand, usually consist of hundreds of people in each group. These tours are in Japanese, but an English audio guide is available to be pre-ordered without charge.
You can view a map of the Imperial Palace grounds here.

One of the most photographed scenes in Japan is of the Nijubashi Bridge, which is part of the private part of the Imperial Palace. "Nijubashi" means "Double Bridge" as there are actually two bridges that cross the moat. The original wooden briges were replaced in the Meiji era with modern stone and iron cast structures. The bridge in the foreground is the "Stone Bridge of the Main Gate", while the bridge in the background is the "Steel Bridge of the Main Gate". "Nijubashi" usually refers only to the "Steel Bridge of the Main Gate" because in the past when the bridge was still made of wood, it had been further reinforced with wooden beams, thereby creating a so-called double bridge.

While much of the architecture is very Japanese-style, there is some buildings that display European architecture, an influence of Westernization brought by Emperor Meiji. This was an important site during both the Edo and Meiji periods, and shows how the Japanese worldview changed from the Edo to the Meiji period.

Websites: http://sankan.kunaicho.go.jp/english/guide/koukyo.html, http://www.gojapango.com/tokyo/imperial_palace_tokyo.htm, http://www.env.go.jp/garden/kokyogaien/english/point01.html