A cortiço is a collective dwelling of little houses and rooms rented to poor people, around a yard with washtubs and taps, common in nineteenth-century Rio de Janeiro. In The Slum, it is the São Romão Tenement, which João Romão builds next to Miranda's townhouse.
The book describes the place with numbers and prices. Once the little houses are ready, "the immense tenement held ninety-five little houses," and João Romão puts up in front a wall ten spans high, crowned with broken glass, with a gate in the center. "The little houses were rented by the month and the washtubs by the day; all paid in advance," and each tub, with water, cost five hundred réis. The tenement's women residents had priority and did not pay to wash, and the place became "a great laundry, bustling and noisy."
Those who live there are laundresses, workers and working people: "the quarry employees all preferred to live there, because they were two steps from their duty." In time the tenement becomes packed, and the narrator tells that "the cocoons subdivided into cubicles the size of graves" and that yellow fever takes two Italian residents.
Outside the book, tenements were a matter for doctors and authorities, who considered them a hotbed of disease. In 1893, three years after the novel, the city government tore down the most famous of them, the Cabeça de Porco, in downtown Rio. Within the book, the Cabeça-de-Gato is the rival tenement of the São Romão. For what the name of the book means, see the question about the title.