Day and night they go about it upon the walls thereof: mischief also and sorrow are in the midst of it.
Day and night they go about it - at all times.
Upon the walls thereof ... in the midst of it - in all the space of the city, alike in the exterior circumference and in the interior area. "They" - namely, violence and strife - "go about it" (Psalms 55:9), as warriors stalking round its ramparts.