The Imperial Palace (皇居) used to be the site of Edo Castle. Located in the Chiyoda ward of Tokyo, it consists of a large park areas, moats, walls made from unbelievably large boulders, guard houses, and classic Japanese buildings. It sits in the very center of Tokyo.
The inner part of the castle is the residence of Japan's Imperial Family so it is generally not open to the general public. But there are two other parts of the grounds that are: Kokyo Gaien, the large plaza in front of the Imperial Palace, and the East Gardens (皇居東御苑). While not the perfect place for the intrepid urban meditator (it can be busy and lacks the curated corners tucked away from the crowds), it is so large and central that it made the list.
Open 9am - 5pm
Closed on Mondays & Fridays
The Kokyo Gaien is a ~10 min walk from Tokyo Station. The East Gardens (Otemon entrance) are near the Otemachi Station on the Chiyoda, Tozai, Marunouchi, Hanzomon and Mita Subway Lines.
Japan, 〒100-8111 Tokyo, Chiyoda, 1−１