Forewarned is forearmed ! Do not expect to see the beautiful gardens you have seen in Hamarikyu, Koishikawa, Rikugien, etc. Kyu Iwasaki-tei is actually the manor/mansion of the Iwasaki family who founded the renowned Mitsubishi Group of companies. While touring the manor please remember that photography is prohibited & you have to walk around with your shoes in a plastic bag which is provided.
It basically comprises of a Western style manor designed by Josiah Conder, and a Billiard house adjacent to it which is designed to resemble a Swiss Chalet and a Japanese house connected to the manor. All are completely made of wood. The garden is little more than a lawn.
Those interested in interior design & architecture, would thoroughly enjoy the visit because the fireplaces, ceilings, columns, banisters, wallpaper, balustrades, etc. are pretty much intact. The opulent western style & the simple, minimalist Japanese style add an interesting contrast.
Mitsubishi has a lot of art & antiques in their corporate museums so, it would be nice if the Metropolitan Government could work something out and place more artifacts in the unfurnished manor to highlight the industrial & cultural changes Japan was going through at that time.
A word of advice, please ensure your socks aren’t torn because you have to take off your shoes when you enter the building. And, if you are visiting during winter you better carry an extra pair of warm socks because the floors are cold !
Here is the link in English for more information ~ http://teien.tokyo-park.or.jp/en/kyu-iwasaki/