I received the following question:

Jeremiah 51:44. What’s the “wall”?

44. I will punish Bel in Babylon, 
And I will bring out of his mouth what he has swallowed; 
And the nations shall not stream to him anymore. 
Yes, the wall of Babylon shall fall.

As you probably know, in those days cities were protected by large walls surrounding them, which kept invading armies out of the city. Thus, the wall was the primary defense of the city. When the wall fell, the city was defenseless. Thus, he speaks here of Babylon’s defenses utterly failing.