Ffordd Pen Llech is the steep lane dropping from the castle side of Harlech toward the lower town. For a while it held the Guinness World Records title for the world's steepest street, a claim later disputed and reassigned, but the argument matters less than the climb itself.
It is a quick, breathy bit of local theatre: stone walls, tight turns, and a gradient that makes the castle feel even higher than it is. Walk down if your knees prefer drama to punishment; walk up if you want to earn your tea.