Sofia
Constantine nearly made it his capital instead of Constantinople—for the mineral springs. Founded by Thracians, conquered by Rome, renamed five times, occupied by Ottomans for 500 years. Now 17.9% of Bulgaria lives atop Roman ruins they're still excavating beneath the metro.
Constantine the Great said it plainly: 'Serdica is my Rome.' He nearly made it the capital of the Byzantine Empire instead of Constantinople—and the reason was water. Sofia sits in a high valley at the crossroads of Central Europe and the Bosphorus, surrounded by mountains, with 15 mineral spring fields pumping 130 litres per second of hot water that Romans turned into public baths, Ottomans turned into hamams, and modern Bulgarians still drink from street fountains. The Thracian Serdi tribe settled here in the 5th century BC for the same reasons: warm water, defensible terrain, and roads running in every direction.
The Romans conquered Serdica in 29 BC and rebuilt it as Ulpia Serdica under Trajan. The city hosted the Edict of Serdica in 311 AD—the first official edict legalizing Christianity, preceding the Edict of Milan by two years. That single administrative decision made Sofia one of Christianity's founding cities. The Byzantines called it Triaditsa, the Slavs renamed it Sredets, and finally in the fourteenth century it became Sofia, after the Church of Saint Sofia—'wisdom' in Greek. Each conquest renamed it; none abandoned it. The mineral springs, the crossroads, and the valley that shelters without imprisoning made the site too valuable to leave.
Five centuries of Ottoman rule (1382–1878) made Sofia a Muslim administrative centre with mosques, bazaars, and the Banya Bashi Mosque (1576), built directly atop a thermal spring. Liberation in 1878 triggered rapid modernization: Bulgaria's new government chose Sofia as its capital and imported Viennese architects to build a European city over the Ottoman grid. Communist rule (1944–1989) added Stalinist housing blocks and the enormous Palace of Culture. The Soviet Army Monument, a Cold War landmark, was finally dismantled in December 2023.
Modern Sofia houses 1.28 million people—17.9% of Bulgaria's population—in a city of 500 square kilometres. EU membership since 2007 attracted outsourcing centres and a growing technology sector. Metro excavations continue to unearth Roman ruins, including the Ancient Serdica archaeological complex beneath the streets—rated 4.5/5 on Tripadvisor and called a 'hidden gem' that tourists walk over without noticing. Constantine's Rome lies directly beneath the shopping district, still fed by the same springs that drew the Serdi three millennia ago.