Walking London Wall

London's Roman Wall is part of the fabric of the city, both literally and metaphorically. As a geographic barrier, 2,000 ago this half-circle of fortification, bounded by the river Thames, set the framework of what is now the City with a capital C – the storied Square Mile, where in recent years financial empires have risen and crumbled in dramatic fashion, echoing the turbulent fortunes of the civilizations that ruled and fought over this pocket of land, from Romans to Saxons to Vikings to medieval monarchs and on into modern history.

After Hadrian's Wall, London's City Wall was the largest building project undertaken by the Romans in Britain. Unlike that great landmark, which is both more visible and high profile as a destination, the remaining fragments of London Wall can seem swallowed up by the volatile topography of a city whose urban landscape is constantly changing. It's hardly a new phenomenon – in earlier centuries Roman foundations were often topped with medieval brickwork and then finished with a further veneer of 19th century additions. Today, sections of the Wall can be discovered in the most contrasting of settings – as a pile of masonry in an old pub's beer garden, or preserved behind glass in a recently built financial head office.

The Wall's Disappearing Act

Often, key developments in the growth of London – such as the burgeoning trade with the New World that saw demand grow for the city's port – had a direct effect on the fortunes of the Wall. In that particular case, the ancient Roman gates were eventually destroyed – all bar one – in order that goods could travel in and out of the city more easily.

These gates live on today in street and station names such as Aldgate, Newgate, and Bishopsgate. The success of London sometimes meant bad news for the Wall, but the opposite could often be true; whole swathes of the Wall emerged phoenix-like from the Great Fire, which razed many poorly constructed buildings and revealed the enduring ancient structures beneath. However, much of the Wall was then destroyed in the 18th and 19th centuries, its bricks reappropriated into new buildings. Into the 20th century and German bombing during the Second World War unearthed sections of the Wall that had long been hidden from view.

Fact and myth often intertwine. In Walking London Wall, Ed Harris explains how Daniel Defoe, writing about the plague, recorded the commonly-held belief that the disease would not breach the Wall. Historical figures show some truth to this superstition – the number of deaths recorded outside the walls was far greater than within. The Wall often played backdrop to some of the city's most dramatic chapters – debtors' prisons, gallows storeyards, churchyards and great fortified towers all sprouted around it like symbiotic plants in a forest ecosystem. The heads of traitors were impaled on it and ghostly dogs rumoured to walk along it.

Harris has produced a dense and colourful account, sometimes too detailed to serve as a walking companion, despite claiming to be suited equally to the armchair traveller and the intrepid explorer. But close reading brings rewards, as this is a book rich with anecdotes both historical and contemporary (Harris encounters many bemused and amused civil servants, barmaids and security guards as he sniffs out the traces of Roman remains in the city's buildings). His enthusiasm for the subject is infectious – he describes a wander around the Wall as "arguably the best value free day out in the capital". In many ways the story of the Wall is the story of London itself, and this book a fascinating though sometimes demanding account of an epic slice of history.