This is a Chidorigafuchi. It's conveniently located at the center of Tokyo; right next to the imperial palace. You have a great view of the water fosse and stone walls of the great Edo Castle, currently the Imperial Palace).
Chidorigafuchi is a very popular hanami spot. It’s a moat around the Imperial palace, and Alongside of it are so many sakura trees. Along the moat, there are parks, streets, and passes, so you can enjoy sakura blossoms from anywhere you like.
Chidorigafuchi National Cemetery is the secular Japanese cemetery for war dead, near to the Imperial Palace and Yasukuni Shrine in Tokyo, Japan.
(Reference: wikipedia, wikitravel)