but could not even sustain Some casual shout that broke the silent air, (lines 12-13)
With the image of the falling tower, Wordsworth asks us to maybe be a little more humble in our opinions. He pokes fun at the tower, image of war and military might, which "could not even" sustain a shout at the end of its life. Maybe the tower would also be a good symbol of our sense of certainty over what is true at any given point.
Or the unimaginable touch of Time. (line 14)
For the Romantics like Wordsworth, you could not understand anything without using your imagine as well as your reason. Since we cannot imagine time, we don’t really understand the truth of time either.