The one thing you can't miss when going to Xi'an are the City Walls. I'm not just saying it's awesome and a place you must visit, but you literally have to go past them to go into the ancient city of Chang'An, as Xi'an was known in the ancient days.
Just like many other ancient cities in China and Europe, a strong wall surrounded by a moat were built to protect the capital of the Chinese Empire. It all started as a big pile of dirt in the 14th century that evolved into this you see today, fully equipped with drawbridges, archer stations, watchtowers, tunnels and several other defense systems.
Modern Xi'an surrounds the city walls, that today only define the ancient city.
Every 200 years or so the wall went through a major refurbishing, the most recent one being in 1983. Basically what you see is more or less how it used to be in old times, but completely rebuilt in the modern days.
During spring festival, when I went, the top of the wall had dozens of intricate lanterns, divided into groups representing several countries around the world. They would light up at night, forming a beautiful constellation of lights. This kid was blowing soap bubbles and running around through the lanterns.
The walls encompass an area of 36 square kilometers (you calculate the perimeter) so it takes quite a while to go around the whole thing. In certain spots you can rent bikes if you want to speed through.
The omnipresent Bell Tower of Xi'an as seen from the wall. It seems like everywhere you looked, there it was.
This is a view of the inside part of the wall so you can see how big it is. Up on the wall are the lanterns I was describing in the first photo.
Bonus: Hua Pagoda of Bao Qing Temple
From the top of the wall I was able to see this pagoda. It took me some time online now to figure out what it was.
The Bao Qing Pagoda was built in 604 at a different site. This one was moved here in 1451 during the Ming Dynasty.
You can see a little Buddha in the "window" of the pagoda.