The story opens with very vague references to what is going on. We learn that it is the Spanish Inquisition, where many people who wouldn't convert to Catholocism were arrested, tortured and killed. Somehow, the narrator himself has been captured. We don't know why, or by whom. He just mentions judges. He is groggy, he can't see their faces clearly, it's in a dark room with candles and curtains, and then all goes black before he awakes in his prison. Being totally dark in the prison, he has to explore and discover for himself the horrors that await him there.
Not having specific details increases the suspense because it leaves the reader free to fill in gaps with their imaginations. And, our imaginations tend to come up with much better, much more frightening scenarios than if we had been given the facts. Who are these judges? Are they evil men bent on torturing the narrator? Why is there? Did he do something awful and horrible? What will his punishment be? We imagine all sorts of horrors. The narrator himself gives into these frightening imaginings himself. Then, when he is in the pit, we have no idea where he is or what is going on. The darkness appeals to everyone's stress levels--no one likes being in the total dark. The suspense is taken up even further; we, like the narrator, feel lost, confused, and imagine awful things, just as the narrator himself does.
The human imagination is a powerful force for suspense--it fills in details and can think of tales and possibilities that are far beyond what is even possible at times. If authors just give a few details, it forces the reader to use his imagination, which is quite powerful, to create suspenseful possibilities. Not knowing engages the mind in powerful guessing games, making us question and become completely involved in the story. I hope that those thoughts helped; good luck!