León's Roman walls are among the best-preserved in Spain. Built by the Legio VII Gemina in the 1st century AD, substantial sections still stand to their original height along the northern edge of the old quarter. The walls were later reinforced by the medieval kings of León and again in later centuries, so what you see is a layered structure — Roman masonry at the base, medieval additions above.
The best-preserved stretch runs along Calle de la Caño Badillo and Avenida de los Cubos, where you can see the characteristic rounded towers at intervals. Free to walk past; some sections can be climbed.