The city walls is a must for visiting Xi’an. You can rent a bike and ride around the whole wall in about 2 hours and cost Y40 for admission plus Y20 to rent the bikes (Y200 deposit).
It’s a easy ride with alot to see along the way. Try to stop every 5 to 10 minutes and enjoy the views which almost always contain a part of old and a part of new Xi’an.
Most of the wall is restored or rebuilt and if you don’t feel like riding you can walk the wall in about 4 hours.
Be aware there is not much shade and places to rest, so if its hot or raining it may be worth waiting till it cools down or the rain stops.