In the article Raindrop from the book What If?, there is a sentence:
On the horizon to the southwest, ominous-looking clouds begin to appear. The towers build as they draw closer, the tops spreading out into an anvil shape.
What does it mean? Draw closer means approaching, and tower as a noun has no meaning other than a building. I understand that this might be a metaphor, but then why it is in active voice? Doesn't it make more sense in saying:
As they (the cloud) draw closer, the towers are built.
And there is more than one tower. Shouldn't it have only one? I am thinking about the tower of the tornado: