The Great Wall of China is the world’s undisputed greatest defensive wall. But there are many other defensive walls, both within and outside of China. The following Wikipedia articles provide a good introduction.
Defensive walls other than the Great Wall of China include the Berlin Wall, Hadrian's Wall, city walls, and more. City walls are discussed here.
There are more than one Great Wall in the world. This one is not so well-known but as well a remarkable frontier building.
Short for everyone who has not heard about this one: From the Rhine to the Danube, built by the Romans in what is today Germany from 83-260 a.C. at a length of about 570km. More to read about and maps as well on wikipedia.
For I can reach this frontier wall easily within some hours of train I started a walk-thru in may 2008. Here some photos of this walk:
Only in the woods the wall can still be seen.
The trail mark ...
... and sometimes the view openes to the Rhine valley.
The apple trees blossomed.
The defense structure consisted of a moat, a rampart and a palisade. This one has passed away but has been reconstructed on some places.
Watchtowers had been an integral part of the defense system. They are about 500m apart. The reconstructed towers do very often not fit the original ones. Those had been painted with white colour and both could be seen from far away and expressed the technical predominance of the Romans as well.