To understand a place like Luri­gancho, it’s best not to dwell on words like prison or inmate or cell*, or on the images these terms generally connote. The 7,400 men who live in Lurigancho, Peru’s largest and most notorious penal institution, do not wear uniforms; there is no roll call or lockdown or lights­out. Whatever control the prison authorities have inside Lurigancho is nominal. They secure the gate to the prison, and little else.

The complex's twenty housing blocks can be divided roughly into two sections: the better-­off inmates live in El Jardín (the Garden), the odd­ numbered blocks. The greenery with­ered long ago, but the name and its cachet have remained. Many residents carry the keys to their own cells and are free to wander the grounds as they wish, though some prefer not to leave the relative calm of their territory. The other side of Lurigancho is known as La Pampa (the Plain), the even­ numbered blocks, home to thousands of accused murderers and petty thieves. The density here can be twice that of El Jardín, the conditions unsanitary and often violent.

Lurigancho is only a few miles from the center of Lima, Peru’s capital and largest city, and it remains connected to the life of the city. La Pampa is organized by neighborhood, each block corresponding to a different district of the capital. The blocks constitute an imaginary map of Lima’s criminal world—one for San Martín de Porres, another for La Victoria, another for San Juan de Miraflores, and so on—each section serving as welcoming committee, support group, and finishing school for the young delinquents who have the misfortune to arrive there.

Separating El Jardín from La Pampa are a high brick wall and a narrow alley known as El Jiron de la Unión, whose namesake was once the most aristocratic promenade in Lima’s colonial center. The prison version is an open-­air market where one can get a haircut and buy soap, batteries, razor blades, old T­shirts, drug...