We see the statue of Ozymandias in the desert. But only two trunkless legs of stone and a shattered visage remains. On his face can be seen expressions of arrogance and contempt. The sculptor has portrayed these expressions with much detail. 
But, nothing else remains beside the broken remains of the statue. This is how the poet brought out the futility of power, and richness.
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The poem brings out the transience of man and his foolish urge to immortalize his name. man is too feeble to withstand the ravages of time. hence wisdom lies in leading a modest and unpretentious life and later on withdrawing from the world with a sense of resignation.