The extended simile in the passage is: As when a circling wall the builder forms, Of strength defensive against wind and storms, Compacted stones the thickening work compose, and round him wide the rising structure grows.
A simile is a figure of speech that compares one thing from another thing of a different kind. It adds meaning to the text because it emphasizes the:
a. Strength of the wall - which is likened to being a barrier that can be used as protection from wind and storms.
b. Size of the structure - which was built with hours of hard work and construction materials that can withstand such great forces of war.