Daphne Yeo
Google
Looks like a less grand version of the one in Xi'An, this 古城墙 with its unique 4-tier structure, is said to be the longest brick city wall in the world at 35km. I'm not sure how that works against the Great Wall, perhaps the longest complete wall. Entrance fees at RMB30 per person, no pre-booking required, in fact it's probably the least crowded attraction all week. There's a small museum at the top that provides the history of this wall, from how it was constructed to how it was restored. Along the path up, look down and you'll find some original bricks with engraving, indicating the person responsible for making and laying the bricks in that section back then.
You also get a superb panoramic view of Xuanwu Lake from this vantage point.