The ruined Roman fort at Richborough in Kent contains a vast and mysterious slab of concrete, 30ft deep, which resembles a mausoleum or a strong room. In a fascinating programme based on informed detective work, the chief executive of English Heritage, Simon Thurley, shows how it was the foundation of a monumental arch built between AD80 and AD90. The arch, a symbolic gateway between land and sea, was covered in 400 tonnes of marble and surmounted by an emperor cast in bronze. The bulk and splendour would have reminded everyone for miles around what it meant to belong to the Roman Empire.
