How does the flashback of Jonas’s friend Asher help establish the setting?

Poe's ability to build up tension is pretty much unparalleled. In this short story we have a man who is trapped in a cell, helpless and tied down while a swinging pendulum threatens to slice his heart open. It's an impossible situation. From the beginning all we feel is desperation. Perhaps the most impactful element is the pendulum itself, but to me, the climax of the suspense is at the finish line: when he's burning while the walls start closing in on him, because at that point we most fear his end is closer than ever as he might fall into the pit.

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