Why it looks like this

Tijuana exists because of the line: founded in 1889 just south of the boundary, it grew as the crossing city — casinos in the Prohibition years, factories (maquiladoras) after the 1960s, and always the gate itself: San Ysidro, one of the busiest land border crossings in the world.

The geometry is stark. North of the line, San Diego's suburbs keep polite distances; south of it, Tijuana's streets run to the fence itself. Beyond the flat river zone, the city climbs into canyon country, where self-built neighbourhoods follow the ravines — a grid pressed against a wall, spilling into hills.