The East Gardens of the Imperial Palace are open to the public. It’s a very picturesque place where you will find the original site of Edo Castle’s innermost circles of defense, the honmaru (“main circle”) and ninomaru (“secondary circle”). Today, only the the moats, walls, entrance gates and several guardhouses still exist. You will also find a marker where the 47 ronin story started.
To be honest, there is really isn’t a lot to see there, but where the old castle tower was located you can get a great view of the Marunouchi skyline. The East Gardens are a place to just enjoy a walk. Where the old buildings of the secondary circle of defense (ninomaru) at the foot of the hill, you will now find a nice Japanese-style garden. And lastly just inside the front gate you there is a small art museum where Emperor Hirohito’s art collection is displayed.