Sofıa

Serdica is my Rome, Constantine the Great is said to have declared, weighing whether to build his new capital on this Balkan plain rather than the Bosphorus. He chose Constantinople instead, and Sofia never quite got its turn at empire.

Most European capitals keep their Roman bones behind glass in a roped-off Old Town, but Sofia has no Old Town at all - its Roman city, Serdica, sits directly beneath the modern one, so commuters cross two thousand years of history on the way to work without breaking stride.

Visitors expecting a walled medieval quarter tend to circle Alexander Nevsky Cathedral's golden domes and call it a day, missing what lies underfoot everywhere else.

VoiceMap's self-guided audio tours take you down into it - through the excavated streets beneath the Largo, across the Square of Tolerance where a mosque, a synagogue and a cathedral share one corner, and along the mineral springs Roman legionaries once soaked their feet in.

Sofia doesn't have an old town. It has old floors. Lace up, plug in, and start digging.