Although called a castle, Nijo Castle (or Nijo Jo) is not actually a castle anymore – the most of the castle was destroyed in the 18th century. It originally was the Shogun’s western residence, and after then was an Imperial Palace, although it was eventually donated to the city of Kyoto and is open to the public. A lot of it is original, which is nice (and rare, even in Kyoto). It has moats, some gardens, castle walls, and a variety of buildings.
I liked it a bit – it wasn’t crowded at all and had some good scenery, in addition to being very spacious. There’s one building you can enter (it closes earlier than the grounds) but it closes earlier than the castle grounds. The building is authentic, but has some cheesy looking mannequins inside. It doesn’t really have English explanations and parts of it look to be in need of restoration. Still, I enjoyed it. The outside is nice, but it still is outside – not a place to go to in inclement weather.
The site overall is OK, but is hampered by the location – it’s a bit far from other sites in Kyoto. If you’re into castles, there’s also Hikone Castle and Himeji Castle (Hikone is much closer than Himeji to Kyoto, but Himeji is very impressive and is a World Heritage Site by itself). Also, an Izumiya (Japanese supermarket) is rather close to the castle.
Compared to other places in Kyoto, the hours of the castle are a bit limited; it is open from 8:45 to 4:00 (for entrance, you must leave by 5:00). It is closed Tuesdays in December, January, July and August. It is also closed from December 26th to January 4th. It costs 600 yen for adults, 350 yen for middle and high school students, and 200 yen for elementary school students.
To get to Nijo Castle, the best way is to take the subway to Nijo-jo Mae station. It can also be reached by buses 9, 50, and 101 from Kyoto Station and buses 61, 62, and 63 from Shijo Kawaramachi. Note that those are just the closest buses, and not necessarily an exhaustive list (as the castle grounds are quite big, there are many buses).