The City Wall is fantastic, and well worth visiting. The thing to do is to rent a bike and go all the way around. You only get the bike for 100 minutes, though, so keep an eye on the time. I thought there would be no problem, but I stopped so much for pictures that I made it bake with only eight minutes to spare, and that was after going as fast as I could for the last 1/3 of it. Should be fine for reasonably healthy people if you pay attention, and definitely worth a visit. I started at the South Gate (by the Drum and Bell Towers) but I think you can rent a bike at a few other gates as well.
If you own or manage Xi'an City Wall (Chengqiang), register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.