The gens Ulpia was a Roman family that rose to prominence during the first century AD. The gens is best known from the emperor Marcus Ulpius Trajanus, who reigned from AD 98 to 117. The Thirtieth Legion took its name, Ulpia, in his honor. The city of Serdica, modern day Sofia, was renamed as Ulpia Serdica.

Sofia is the capital and largest city of Bulgaria. It is situated in the Sofia Valley at the foot of the Vitosha mountain, in the western part of the country. The city is built west of the Iskar river and has many mineral springs, such as the Sofia Central Mineral Baths. It has a humid continental climate. Being in the centre of the Balkans, it is midway between the Black Sea and the Adriatic Sea and closest to the Aegean Sea.