After yellow fever drove elite families to northern barrios like Palermo and Belgrano in the 1870s, San Telmo’s narrow colonial streets became an area where impoverished immigrants could find a foothold in conventillos, abandoned mansions where large families filled small spaces—often a single room.

Today, it’s a mixed neighborhood where conventillos still exist but young professionals have also recycled crumbling apartment buildings and even industrial sites into stylish lofts. Famous for its Sunday street fair, a favorite among Argentines and foreigners alike, it’s one of the city’s best walking neighborhoods.

While colonial Spanish law dictated a city plan with uniform rectangular blocks, in practice things were not quite so regular. North–south Calle Balcarce, for instance, doglegs between Chile and Estados Unidos, crossing the cobblestone alleyways of Pasaje San Lorenzo and Pasaje Giuffra.

The Casa Mínima (Pasaje San Lorenzo 380) takes the casa chorizo (sausage house) style to an extreme: Now open to the public as part of tours of the nearby El Zanjón de Granados (Defensa 755, tel. 011/4361-3002, www.elzanjon.com.ar), the width of this two-story colonial house is barely greater than the an average adult male’s arm-spread.

To the east, on Paseo Colón’s Plaza Coronel Olazábal, sculptor Rogelio Yrurtia’s Canto al Trabajo (Ode to Labor), a tribute to hardworking pioneers, is a welcome antidote to pompous equestrian statues elsewhere. Across the avenue, the neoclassical Facultad de Ingeniería (Engineering School) originally housed the Fundación Eva Perón, established by Evita to aid the poor—and her own political ambitions. Three blocks south, beneath the freeway, the so-called Club Atlético was a clandestine torture center during the Proceso dictatorship; it is now a memorial park.

San Telmo’s heart, though, is Plaza Dorrego (Defensa and Humberto Primo), site of the colorfully hectic weekend flea market. A few blocks south, in a cavernous recycled warehouse, the Museo de Arte Moderno (Avenida San Juan 350, tel. 011/4361-1121) was closed for remodeling but was to reopen in late 2010.