Q:

I don't understand the usage of "out of"

The burg was surrounded by two belts of walls: the first ran around the castle (the northern side of this belt is still well preserved today), while, out of the second belt, which extends around the citadel, only the main access gate has left. 

What do you think about "out of" in that position? 

A:

The burg was surrounded by two belts of walls: the first ran around the castle (the northern side of this belt is still well preserved today), while, out of (= from) the second belt, which extends around the citadel, only the main access gate has been left. 
out of

Indicating the source or derivation of something; from.