The chink in the wall.  

Walls separate, divide,

restrict, enclose and protect: 

Drystone wall:  

walled in

wall-to-wall:  

wall of silence

walls have ears:  

wall of sound

off the wall:  

walled off

hit the wall:  

walled up

go to the wall: 

walls and bridges

drive someone up the wall

between you, me and the wall