Among the many visitors to Niji-jo castle, relatively few make their way to Honmaru Goten (本丸御殿 - a group of castle buildings and gardens within Nijo-jo castle) located at the far end from the front entrance.
In fact, Honmaru was located at the very center of the castle and served as the control room at the times of many wars. Unfortunately, the original Honmaru buiding was destroyed by a big fire in the middle of the Edo period. The current building was formerly a residence of a court noble clan called Katsuranomiya (桂宮), located at the Gosho (御所) palace, and later transferred to Nijo-jo castle in the middle of the Meiji period. The building is very classy and beautiful. If you look up the Honmaru building, you will be able to feel the everyday life of court noble people of Japan.
I strongly recommend you visit there during your stay in Kyoto.