I visited the Zhonghua Gate during my first day in Nanjing. It is easily found and a popular tourist destination for domestic and foreign tourists alike. One of the 13 original gates of Nanjing's City Walls, this is a massive structure and it took me about 30 minutes to assimilate just the sheer size of it. There is a small museum inside that describes the castle in the context of the Ming Dynasty and has a small diorama of the area with a collection of ancient bricks used to build the gate; English signage otherwise was sparse. I took a tour book along that described the inner citadels, portcullises, and ramps that allowed soldiers and horses quick access to the ramparts. As a bonus, you can access the city walls from the tower platform on top and even rent a bike (!) for transit along the Ming city walls.
Overall this was worth the trip and really put the Ming Dynasty's stamp on Nanjing into context. An audio guide would have been great but this is also a place that you can appreciate just from a physical/aesthetic standpoint. Recommended.
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