“weak.” An idiom. The literal Hebrew is “their inhabitants—small in hand.”
“like the plants of the field and like the green vegetation.” The ancient and powerful cities are here compared by the figure simile to the grass of the field and green vegetation, which is very short lived.
“grass on the housetops.” The houses had flat roofs that were often surfaced with hardened mud, which grew weeds. But the weeds had no depth of earth and were not watered, so during the dry season they were quickly scorched and died.