I was just aimlessly exploring the city when I saw this imposing torii (gate). I checked what it was, and I was pleasantly surprised that it turned out to be the entrance to the Yasukuni Shrine - a shrine I have seen many times on international tv. This is the shrine that always attract a lot of media attention whenever China protests the visits made by local and foreign dignitaries.
Here's the Wikipedia article on the shrine, lifted from the press release for the movie Yasukuni.
Yasukuni Shrine (靖国神社, Yasukuni Jinja is a Shinto shrine located in Chiyoda, Tokyo, Japan. It is dedicated to the kami (spirits) of soldiers and others who died fighting on behalf of the Emperor of Japan. Currently, its Symbolic Registry of Divinities lists the names of over 2,466,000 enshrined men and women whose lives were dedicated to the service of Imperial Japan, particularly to those killed in wartime. It also houses one of the few Japanese War Museums dedicated to World War II. There are also commemorative statues to mothers and animals who sacrificed in the war.
Yasukuni is a shrine to house the actual souls of the dead as kami, or "spirits/souls" as loosely defined in English. It is believed that all negative or evil acts committed are absolved when enshrinement occurs. This activity is strictly a religious matter since the separation of State Shinto and the Japanese Government in 1945.
The priesthood at the shrine has complete religious autonomy to decide to whom and how enshrinement may occur. They believe that enshrinement is permanent and irreversible. According to Shinto beliefs, by enshrining kami, Yasukuni Shrine provides a permanent residence for the spirits of those who have fought on behalf of the emperor. Yasukuni has all enshrined kami occupying the same single seat. The shrine is dedicated to give peace and rest to all those enshrined there. It was the only place to which the Emperor of Japan bowed.