Most of Edinburgh's Old Town is a ridge; this walk is about everything that fell off the sides of it. The Grassmarket sits in a hollow directly beneath the castle rock, and the streets around it — Victoria Street's curve, the West Bow, the sunless Cowgate — are the city's basement. Two and a half kilometres, mostly downhill then a climb out.
Start at the top of the Mile and drop off the ridge past the Camera Obscura onto Victoria Street, the stepped, curving terrace everyone photographs. Walk the upper level first for the view down the bend, then the shopfronts below. It pours you into the Grassmarket, the wide square with the castle rearing up behind — sit here with a drink and look up. The West Bow dog-legs east into the Cowgate, the canyon street that runs in permanent shadow under George IV Bridge and South Bridge, at the literal bottom of the city.
Climb out up Cockburn Street's Victorian curve, which delivers you to Waverley with the Old Town stacked overhead. You leave the under-city the only honest way: by climbing.