When someone makes reference to a familiar story or cultural reference, such as Ulysses and Achilles from Homer's "The Odyssey", it is considered an allusion. 
        
             
        
        
        
The lines that describe the decline and fall of the city are the following:
- These wall-stones are wondrous —  calamities crumpled them, these city-sites crashed, the work of giants  corrupted. 
- The roofs have rushed to earth, towers in ruins.
- The halls of the city  once were bright: there were many bath-houses,  a lofty treasury of peaked roofs, many troop-roads, many mead-halls  filled with human-joys until that terrible chance changed all that.
- Days of misfortune arrived—blows fell broadly—
death seized all those sword-stout men—their idol-fanes were laid waste —the city-steads perished.
-  This place has sunk into ruin, been broken  into heaps,
 
        
             
        
        
        
Bcz they are exaggerating 
        
             
        
        
        
The literary device presented in Ophelia's song is "<em>simile</em>".
Simile is a figure of speech that compares to things with no necessity of a huge explanation. This happens when you compare two things.
In Shakespeare's masterpiece, Ophelia's song was considered to be very revealing because at that time, the way she spoke or what she said throughout the song was socially unacceptable due to the fact that she was an unmarried women and talking so openly was an issue.