I have walked around, beside, and on top of the wall both day and night. I rode a bike on top which is a "must do" while you are here. The 12.4 km is not far but the Wall is uneven regardless of it's high maintenance and the bikes have seen better days but the trip it was well worth it. I will certainly do it again a few more times before I leave. You can rent a bike from any of the entrances at each main gate. You need a 200Y (C$40) deposit and the cost for 2 hours, which is all you need, was 46Y (C$9). There is no issue with getting your deposit back. Just do not loose your payment slip.
I went to the East Gate as it the closest gate to my house, about a 20 minute walk. I paid my 54Y (C$11) entrance and up I went. The East Gate entrance is a bit tricky to find. It is "inside the wall" but you have to make sure you cross at the crosswalks if you do not want to get killed. There is quite a bit of traffic and if you decide to cross and walk along the inside Island because it is quicker you have to be well aware that you will constantly be in a blind spot to on coming traffic. The second thing to remember is that Chinese drivers do NOT follow the rules of the road so if hugging the curb in what you think is the safety of the 12 inch white line think again. If you get hit YOU will be the one at fault. There is NO arguing that point. The driver could have been drunk, speeding, texting and eating a sandwich while driving with his feet but you will be to blame because of where you walked.
An important note. At the South Gate, which is the main tourist gate, you will be asked to get off your bike. You give your bike to an attendant who in return gives you a bracelet. You then walk for 20 minutes or so along the wall in this busy area. A great safety feature if you ask me. I was initially confused by what they were asking but a nice Dutch couple told me what was going on. They had the same misunderstanding that I was having and decided to "pay it forward". You pick up a new bike on the other side of the busiest area of the wall and you continue your journey. My new bike was worse than my previous but I only had a short trip to make back to the East Gate. It was slow and steady as I was in no rush.
I have walked around the outside of the City Wall numerous times both day and night. The wall is surrounded by a moat as all ancient cities were. All along the wall are walking trails, exercise equipment including Ping Pong Tables and lush greenery. You can also find people singing, dancing, playing badminton, doing Tai Chi or just hanging about chatting and enjoying themselves.
Although it may seems similar each section of the City Wall is a bit different. The South Gate is the main tourist and celebration gate. It is surrounded by walking streets with bars, restaurants, sides streets and artisan markets. The North Gate is where you can find the main train station and onward buses. It is here where I took the bus to Lishan and the Terra Cotta Warriors Museum. It is always a beehive of activity with travelers coming and going and is the place that if you were to be robbed or pick pocketed this would be the place. That being said the police presence is very strong and quite intimidating. The West Gate moves into a newer residential and business district. You can find a few historical sites here along with a marvelous temple in the inside of the walls north west corner. The Eastern Gate which is the closet to me is considered a "local area". It is busier than the South Gate but with zero tourists, well except for me. There are local restaurants, the parks and plentiful but if you are to get killed by a car this is your spot. There is a shop here that is my "go to" for a cold beer after hanging out at the wall.
My suggestion is for you pick an area and explore it. Go slow and take a look around you. Stop, take a few photos but do not spend the entire time looking through a tiny screen. Watch what people are doing. Look at how they are embracing the joy of the situation they are a part of. It is a great example of people "living in the moment". Walking around the wall gives me a sense of calm so I embrace it when ever I can.
It is at night when the wall takes on a whole new dimension. The lighting on the wall is perfect. The ramparts and towers are given just the right balance of color and luminosity. The trails below have soft lights on the paths and the physical walls have spot lights running up giving it a perfect balance. To say it is imposing is an understatement. When you are on the wall at night there are dark areas where you walk and with many people around the atmosphere seems to create a silent mood. You find yourself surrounded by people all walking in general silence enjoying the experience. I was expecting someone to jump out of the shadows and start killing us like a great horror movie.
I went up on the wall one day all excited about taking sunset photos. I wandered to the Western wall and my excitement disappeared. Of course there were going to be tall apartment buildings in the way dammit! I stayed positive, what else was I going to do. Thinking I could catch the sun in between building or some other natural vs man made miracle. It did not happen. I was chatting with a few tourists that happened along and we all came to the conclusion that most people in Xi'an have not seen a dramatic sunset let alone sun rise. You need moments like this to take stock at how lucky we are and can be.
Ultimately the Xi'an City Wall is just great spectacle. It gives credence to a spectacular history both culturally and militaristic. It should not be overlooked as a part of your trip to Xi'an. However riding a bike on the wall and visiting the South Gate will not do the experience justice. If you have the time take and extra day and walk around the entire wall. Take your time, stop for lunch and a beer and it may be the best 6 hours you spend in the city. Start late afternoon to experience the wall during the daylight and at night.
When you are just about done go and find a local shop. Buy a beer and find a quiet place to drink it all the while staring at the wall at night. It is a soothing and exhilarating experience at the same time. They buy another beer and snap out of your trance. Go and have some late night fun in the Middle Kingdom.