Look on my works, ye Mighty, and despair!
The common interpretation of the poem is that we're supposed to appreciate the irony of the self-styled king of kings' works not being around anymore. Time, like sand, eventually wears everything out. But I always found it funny that the pharaoh is addressing "ye Mighty,". Is it out of respect or irony that Ramesses II addresses his non-peers this way? The pharaoh not-so gently cues them to despair, so I don't suppose it's out of respect. But the irony is also a bit uncalled for: he already drove the point home about who's in charge with his title. Why "ye Mighty," then?
Unless it's not supposed to be ironic, but earnest. Maybe Shelley isn't picturing a cocky, hubristic ruler, but a wise one. He was, after all, one of the most successful pharaohs of Egypt, in both war and diplomacy. Maybe the pharaoh is trying to send an important message to those who happen upon his works: "It doesn't matter how 'Mighty' you are. I was the greatest of all, and now, look around: all is for naught. Your work, too, will eventually be forgotten." This reflection on the transient nature of power and achievement, to me, is a more profound reason for "ye Mighty" to despair than simply observing the magnitude of his monuments when they were around.