How does the narrator feel about his prison in "The Pit and the Pendulum"?

The simple answer is also the most obvious one: he does not like his prison, as it is a prison. The less obvious answer is a little more tricky. He quickly rules out that he is not dead, but he fears in what condition he might be. He remembers all of the "vague rumors of the horrors of Toledo", and he starts to wonder if they were not rumors at all. Worrying that he would be left to die of starvation or worse, he starts to investigate to the best of his ability, despite the fact that the absolute darkness of his cell prevents him from seeing anything. His mood becomes less frantically paranoid as curiosity takes over and he starts trying to figure out his surroundings. It seems almost that he develops a clinical interest regarding his cell, and although there is still fear of the dangers he faces (such as the pit and the pendulum and the walls literally closing in around him), the frantic fear from the beginning when he had absolutely no idea where he was does not make a reappearance, particularly once the lights come on, allowing him to see his surroundings.

In the end, although he is first incredibly fearful of his unknown prison, once he sees what he is dealing with, he becomes less blatantly terrified and more interested and curious.