Irony
The statue is of course ruined - the legs remain but the body has fallen. The face ('visage') lies on the sand, 'half-sunk' and 'shattered', making it hard to recognise. According to the inscription, which has survived, the king Ozymandias set up the statue to draw attention to his 'works' - but his own face has not survived, let alone the empire he may have once ruled.
The stretching of the 'lone and level sands' in every direction cover any buildings or rich farmland that may have flourished here. However, one survivor beside Ozymandias' words is the sculptor's skill: it is witnessed by the success of the statue in capturing 'those passions' of the king, even when partly ruined. Nameless, it is the sculptor whose works are still valued, just as Shelley's poem survives from his own day.