© Neil Howard (Flickr)

The Tower of London is lit up after dark. It wants to be seen: the biggest, stoniest jewellery case in the world. Today, it's a symbol of British history, but for centuries, it was a monument to real power: a warning to enemies of the state, be they traitors, spies or heretics. Those labels are stories in themselves. The state decides how they apply - and to whom.

The latest commission from Historic Royal Palaces offers a rare after-dark peak inside a building that's been locked up every night for the last 700 years. It prods at the stories we swallow - about the past, but also about the present; all the things we can't verify for ourselves. Given that what we believe determines what we'll do, selling someone a story is a form of control. Spin is power, in other words, and Bristol-based company Anagram's audio experience spins us this way and that.

Two voices on the end of a phone - one female, one male - guide us through the garrets and ramparts, up stone stairs and down cobbled streets, relaying information and imparting instructions as we go. Which of them do we trust - and why?

Nightwatchers is partly a crash-course in reading architecture. The walls of the Tower are built to tell stories - from the fake medieval turrets that the Victorians erected to the replica rooms reconstructed for today's tourists. Others have stories scratched into their surface: names carved into stone by imprisoned traitors and heretics, testifying to innocence or unfaltering faith. Across the river Thames, glass towers stare back at this stone one: a story of transparency instead of impenetrability, but a different kind of power and a story all the same.

Increasingly, the language glitches and warps, so that the past becomes entangled with the present. Heretics become religious fanatics and traitors, extremists. One species of surveillance swaps in for another. We might be seeing differently, but we're also being seen - and, as Nightwatchers continues, being seen in a different light. Anagram aren't afraid to manipulate an audience; something that marries well with both the form and the technology; technology that tells us stories about the world on a day to day basis.

Nonetheless, the 'mission' framework can't help but feel naff and, the further the show pushes us, the more its credibility starts to fray. Despite technology that keeps in step with your pace, Nightwatchers still feels like a conveyor belt experience, as you traipse behind someone else, always aware exactly where you're heading and what's coming next. The best stories spring a surprise.

Nightwatchers runs at the Tower of London from 1 to 20 February.