And in one of those transparent sections of the stone wall I had seen this girl and had naïvely believed that she was moving in a tunnel parallel to mine, when in fact she belonged to the wide world, the unbound world of those who did not live in tunnels

Castel is reflecting on his relationship with Maria. He had spotted her observing a detail of one of his paintings at a gallery and became obsessed with tracking her down and meeting her, convinced that she had been living on a parallel track that was destined to intersect with his. He eventually found her and they became lovers, but she was not a tunnel-dweller like Castel. He could not have her, and his obsession with her importance to his destiny became even worse.

How many times had that damned split in my consciousness been responsible for the most abominable acts? While one part of me strikes a pose of humaneness, the other part cries fraud, hypocrisy, false generosity. While one incites me to insult a fellow being, the other takes pity on him and accuses me of the very thing I am denouncing. While one urges me to see the beauty of the world, the other points out its sordidness and the absurdity of any feeling of happiness.

Here Castel is acknowledging a consistent polarity in his inner monologue. Rather than being dominated by his better side, he seems fairly evenly split. His encounter and obsession with Maria and her unwillingness to conform to his expectations and hopes, however, tips the balance toward his darker side, which keeps growing and growing until he knows it has taken over. Ultimately Castel is unable to regain his mental balance and ends up killing Maria.

while I kept moving through my passageway, she lived her normal life outside, the exciting life of people who live outside, that curious and absurd life in which there are dances and parties and gaiety and frivolity.

Castel the tortured artist is finding it hard to have a normal relationship with a woman of wide acquaintance who lives a normal life that the painter has never experienced. He finds her life not only hard to relate to but also a completely unacceptable fit for his tunnel-world. He had invested so much meaning in meeting and having a relationship with Maria that when he found there were many aspects of her life he could not and would not be sharing, it drove him over the edge. His inner dialogue is chilling, as it alternates between the rational and comprehensible to a darker language where the anguish he experiences allows his demons to take over.