Istanbul

The Serpent of Constantinople

Istanbul is the only city in the world that was capital to two empires without ever changing location: Constantinople and Istanbul tread on each other at every corner of Sultanahmet. You'll count heads that are no longer there, decode a Viking runic graffito carved by a mercenary of the Byzantine imperial guard, and track down a Christian eagle hiding above an Ottoman gate. It's just over 3 km all told, with one gentle climb up toward the Grand Bazaar and the same easy walk back down.