Illustrator Lydia Wood has set herself a challenge that sounds simple and turns out to be anything but: sketch every single pub within the boundaries of the City of London, the Square Mile at the capital's historic heart.

It's a fitting subject. The City's pubs are some of the oldest drinking establishments in the country, several claiming centuries of continuous trade, and they've survived fires, the Blitz, and wave after wave of office redevelopment. Wood's illustrations document their distinctive fronts — narrow timber-framed entrances, hanging signs, livery ties and old-fashioned lettering — the kind of details most commuters rush past on their way to the tube.

The project doubles as an unofficial walking guide. Because the City packs an enormous amount of history into a small area, an afternoon crawl can take in coaching inns that once hosted Dickens, taverns linked to figures like Sam Johnson and his sidekick, and drinking spots tucked down alleys that follow the medieval street plan almost unchanged. Wood's drawings capture the characters of these buildings one by one, giving a sense of how varied the City's pub scene really is — from Victorian gin palaces to quiet backstreet locals that only fill up at lunchtime on a Friday.

For visitors, the appeal is practical as well as aesthetic. The Square Mile can feel sterile on weekends when the offices empty out, but its historic pubs stay rooted in place, and a self-guided route between the most characterful ones is one of the better free ways to spend a day in London. Wood's illustrated map-style documentation makes it easy to pick a handful and plot a loop through the old streets around Bank, Monument and St Paul's.

It's also a timely record. City pubs have been closing at a steady pace as developers convert them into restaurants and office space, so a project that puts every remaining one on paper — and on the internet — is quietly doing preservation work too. If you're the sort of traveller who likes a mission for a city break, 'collect' a few of Wood's illustrated pubs and you'll see a side of London that the big-guidebook crowd mostly misses.

Whether the full tally runs to a few dozen or north of fifty, the charm is in the cumulative effect: a hand-drawn portrait of London's oldest drinking culture, one sketch at a time.