The Romans' Northern Gate is easy to walk past without noticing. Tucked beside Cologne Cathedral, a small stone arch sits at the top of a short flight of steps, mostly ignored by the crowds heading for the Dom. It was one of the two side entrances to Cologne's main northern gate, built at the end of the 1st century AD.

The wall it belonged to was no minor fortification. Roman Cologne was a frontier city, and its Rhine marked the boundary between the empire and ancient Germania. A bronze plate nearby shows what the full gate complex once looked like.

VoiceMap's self-guided audio tours start here, using the gate to open the entire story of Roman Cologne: a frontier outpost founded in 19 BC that grew into one of the empire's most important cities, its remains still scattered beneath the streets today.