Final answer:
Imagery is used in the line to evoke a vivid image of emptiness and transience, highlighting the theme of impermanence.
Step-by-step explanation:
The line "Nothing beside remains. Round the decay" utilizes the literary device known as imagery to evoke a vivid picture of the surroundings in the reader's mind. This imagery is effective in conveying a sense of emptiness and transience, as it contrasts the enduring 'decay' with the now-absent vibrancy that once was. The use of this device accentuates the theme of impermanence and the inevitable decline of even the greatest works or civilizations.