I left out the following sentences on the same theme. Thank you.

1) He also sees that his prison is square and that the wall is covered with iron, with horrible figures, such as skeletons. The floor, instead, is made of stones.
2) The ceiling is thirty to forty feet overhead and is similar to the wall. Hanging from one of the iron plates he sees a huge pendulum that seems to be moving.
3) Then he hears a noise and sees lots of enormous rats coming up from the pit, attracted by the smell of the food. He cannot move because he is now bound to a wooden board by a long strap.
4) Looking up at the pendulum, he realizes that its movement has become wider and quicker and that it is coming down. Its lower part was of bright steel, about a foot long; the horns were upwards and the bottom was as sharp as that of a razor.
5) It hung from a brass rod and it hissed as it swung through the air.
He believes the pit is the destiny the monks had prepared for him. It is typical of hell and is considered as the last instrument of their punishment.
6) He decides to put some meat on the strap that binds him, hoping that the rats will eat it and free him before being killed by the pendulum.

1. Change "instead" to "on the other hand" and keep the commas in place.

2. OK

3. OK

4. Verb tenses?

5. Verb tenses? Also remove "as" before "the last instrument"

6. OK

1) In the story, the protagonist notices several details about his prison. Firstly, he observes that the prison is shaped like a square, implying that the space is confined and limited. The walls are covered with iron and are adorned with grotesque figures, such as skeletons, adding to the ominous atmosphere. On the other hand, he notices that the floor is composed of stones, which may imply a rough and uncomfortable surface.

2) Our protagonist then shifts his attention upwards toward the ceiling. He estimates that it is around thirty to forty feet high and similar in material to the walls, consisting of iron plates. From one of these plates, he notices a large pendulum hanging down, seemingly in motion. This pendulum becomes an important element in the story, as it poses a threat to the protagonist's life.

3) As the protagonist contemplates his situation, he becomes aware of a noise emanating from the pit below. This noise reveals the presence of numerous enormous rats, attracted by the scent of food. Unfortunately, the protagonist finds himself unable to move as he is bound to a wooden board by a long strap. This restriction further heightens the tension and danger he is facing.

4) While the protagonist is immobilized, he becomes increasingly concerned about the pendulum's movements. He realizes that the pendulum is descending towards him, and its swinging motion widens and quickens. The lower section of the pendulum is made of bright steel, approximately a foot in length, and its horns are positioned upward, while the bottom is as sharp as a razor. This description foreshadows its potentially lethal nature.

5) Furthermore, the pendulum hangs from a brass rod and emits a hissing sound as it swings back and forth through the air. The hissing sound adds a sense of danger and impending doom to the protagonist's predicament.

6) Faced with the imminent threat of being cut by the pendulum, the protagonist devises a desperate plan. He decides to place some meat on the strap that binds him to attract the rats. He hopes that the rats will be enticed by the food and will chew through the strap, freeing him from his precarious situation before the pendulum reaches him. This moment highlights the protagonist's resourcefulness and desperation as he tries to outsmart his captors and escape his grim fate.