Truth fails not; but her outward forms that bear The longest date do melt like frosty rime, (lines 7-8)
So what is this "truth"? Does it too undergo change, or does it stay the same the way grass continues to exist after the frost has melted off it?
which royally did wear His crown of weeds, but could not even sustain Some casual shout that broke the silent air, (lines 11-13)
Transience often takes the form of a very gradual change that suddenly results in a breaking point. The tower has retained the same basic form over the years, albeit a little worse for wear, but it has been slowly weakening to the point that a loud noise can bring it down.