Barrabás arrived on a Holy Thursday. He was in a despicable cage, caked with his own excrement and urine, and had the lost look of a hapless, utterly defenseless prisoner; but the regal carriage of his head and the size of his frame bespoke the legendary giant he would become. (1.1)
Though Barrabás is in pitiful shape, the fact that he's still in possession of his some irrevocable source of dignity leads us to draw a comparison between his captivity here and Alba's at the end of the novel. Perhaps the author wants to make the point that imprisonment and physical hardship can't necessarily rob a creature of its integrity.
It bothered her to have to stay locked up within these walls that stank of medicine and age, to be kept awake at night by the moans of her sick mother, always attentive to the clock so as to administer each dose at the proper time, bored, tired, and unhappy while her brother had no taste of such obligations. Before him lay a destiny that was bright, free, and full of promise. He could marry, have children, know what love was. (2.8)
Here, the theme of captivity is linked to gender – the only difference between Férula's situation and that of her brother is that she's a woman and he's a man. Férula's captivity may be her own doing (not all women are held back by their gender in the novel – just look at Tránsito Soto), but she's certainly conforming to social roles imposed upon her by society.
These visits to the zoo holding on to the hand of some conceited spendthrift suitor gave her a lifelong horror of enclosures, walls, cages, and isolation. (9.29)
Alba's fear of enclosures and isolation is foreboding. In this novel, if someone has a "lifelong horror" of something, it's likely going to come up later.
He struggled and shouted so much that they loosened his bonds and helped him to his feet, but when he attempted to leave he saw that the windows had been bricked in from outside and that the door was locked. They tried to explain to him that things had changed and that he was no longer the patrón, but he refused to listen. (12.54)
Esteban's not used to being the one with his hands tied. His physical constraints lead us to think of the invisible constraints that have held back his tenants – constraints like poverty, ignorance, lack of education, and no power to change their situation.
The curfew lasted for two days, which to Alba seemed an eternity. On the radio they played martial music, and on television they showed only landscapes from around the country and cartoons. (13.35)
One of the first acts of the new military government is to establish a two-day curfew, confining the citizens to their homes. The curfew might be considered both a demonstration of the military's power as well as a means of control.
Despite the order to shoot anyone who ventured outside, Senator Trueba crossed the street to attend a celebration in his neighbor's house. The hubbub of the party did not concern the soldiers patrolling the streets because it was a neighborhood where they expected no opposition. (13.35)
The fact that Esteban Trueba can violate the curfew with impunity is indicative of his status, and of the fact that the true purpose of the curfew is to neutralize any opposition the military might encounter.
Soon all the embassies were ringed with barbed wire and machine guns and it was impossible to continue taking them by storm… (13.61)
The barbed wire and machine guns surrounding the embassies suggest that the country itself has been turned into a prison. Ironically, the conservatives, under the rule of the Socialist government, had fled the country in fear of state oppression. Now it's the leftist opponents to the military regime who cannot leave.
He cursed his voluntary imprisonment, and raged impatiently for news of his friends, […] He began to be obsessed by the idea that he was a coward and a traitor for not having shared the fate of so many others, and felt that it would be more honorable to surrender and meet his fate. (13.93)
Pedro Tercero seems to feel that forced imprisonment would be "more honorable," and therefore preferable to voluntary imprisonment.
After a few months Blanca realized that she could not hold him prisoner indefinitely and gave up her plans to reduce his spirit in order to make him her permanent lover. She understood that he was being eaten up alive because for him freedom was even more important than love, and that there were no magic pills that would make him change his mind. (13.93)
Blanca's decision is a commentary on the inutility of imprisonment. She realizes she can't get what she wants by forcefully keeping Pedro Tercero in her home. Other jailers in the novel also fail to accomplish their objectives through imprisoning people – the novel doesn't give a single example of torturers extracting a confession from their prisoners.
She tried to count the days since she was first arrested, but her loneliness, the darkness, and her fear distorted her sense of time and space. (14.18).
One of the effects of imprisonment in the novel, and indeed probably one of its objectives, is to disorient the prisoner. Because it helps Alba to reorient herself in time, it makes sense that writing would serve as an effective countermeasure against this distortion.
The doghouse was a small, sealed cell like a dark, frozen, airless tomb. There were six of them altogether, constructed in an empty water tank especially for punishment. They were used for relatively short stretches of time, because no on could withstand them very long, at most a few days, before beginning to ramble – to lose the sense of things, the meaning of words, and the anxiety of passing time – or simply, beginning to die. (14.59)
Alba's imprisonment in the doghouse recalls the arrival of Barrabás at the del Valle home in the beginning of the novel. In this quote, the author's message is clear: imprisonment and isolation, which is separation from all communication and from time, equal death.