Republic-era architecture, a vast collection, and a reservation system that does not care that you are already in Nanjing. Book it before you book the duck.
The historical galleries are the reason. Go straight there if your slot is short.
It is a museum, not an ancient site. The building has history of its own. The collection is why you are in the room.
Getting there
Metro to the museum stop on the east side, toward Purple Mountain but not on the mountain. It pairs badly with a mausoleum slot the same morning. Give it its own half day.
Staying nearby
Any central hotel. The museum café is not the meal. Leave hungry and eat salted duck in town.