The London wall was a great semicircle arcing north from the river at Blackfriars, east along Moorgate and then back south to the river at the Tower. Not much of the wall itself has survived, but even after all this time the Romans’ layout is still preserved in the pattern of London’s streets.

The walk didn’t follow the whole line of the wall, just the section that cut east of the museum at the Barbican, passing north of the City and then down to the river by the Tower. There were supposed to be little numbered ceramic plaques you could follow, with the first few in the area of the museum itself, which had been built on the site of one of the Romans’ forts. Number one plaque was at the Tower and number twenty-one near the museum, so we were going to have to follow the line counting down, which bothered my sense of neatness, and earned me the day’s first bit of mockery from Linda.

The first few plaques were hard to find in the Barbican’s three-dimensional concrete maze of roads and highwalks—“Like an inside-out prison,” as Linda put it. The first plaque was glued to the wall of a modern bank building; it showed the site of a late Roman city gate, now long demolished. By the time we got there Linda was already sweating. “Is this going to be the story of the afternoon? Crappy little plaques showing where things used to be?”

“What did you expect, Gladiator? . . .”

The next few plaques led us around the perimeter of the old Roman fort. Stretches of the wall were visible in scraps of garden below the level of the roadway. Much of the wall had been built over in medieval times, then uncovered by the archaeologists. The ground level had risen steadily over time; we walked on a great layer of debris centuries thick, a measure of the depth of time itself.

Plaques seventeen to fifteen caused us some arguments, because they were scattered around the ruins of a round medieval tower set in a garden in the shadow of the museum itself. We traipsed over the grass-covered ground, to the water and back, trying to figure out the peculiar little maps that supposedly showed us how to get from one plaque to the next in line.

Plaque fourteen was in a churchyard that turned out to be a little oasis of peace, set away from the steady roar of the traffic. We sat on a bench facing a rectangular pond, bordered by concrete. The wall, with its complex layers of medieval building and rebuilding, stretched its way along the bank opposite us, passing the remains of a round fort tower. I’d brought a couple of bottles of Evian, one of which I now passed to Linda. She had been right about the shoes. My arches were already aching.