drop like the tower sublime Of yesterday, (lines 10-11)
"Yesterday" is a kind of pun. He means that the event literally happens yesterday: "Hey, did you hear about that tower that fell over in the Lake District?" He also means "yesterday" in that metaphorical sense of historical time.
Or the unimaginable touch of Time. (line 14)
Now what’s so unimaginable about time? Old tower gets weaker, then falls over – seems pretty easy to imagine to us. Maybe it’s the particular image of the "touch," which personifies time, that we have a hard time wrapping our heads around.