In which way did ancient Rome influence and unify medieval Europe?

Medieval guilds were inspired by Roman ones.

The Early Middle Ages or Early Medieval Period was the period of European history lasting from the 5th century to the 10th century. The Early Middle Ages followed the decline of the Western Roman Empire and preceded the High Middle Ages (c. 1001-1300). The period saw a continuation of trends begun during late classical antiquity, including population decline, especially in urban centres, a decline of trade, and increased immigration. The period has been labelled the "Dark Ages ", a characterization highlighting the relative scarcity of literary and cultural output from this time, especially in Northwestern Europe. However, the Eastern Roman Empire, or Byzantine Empire, continued to survive, and in the 7th century the Islamic caliphates conquered swaths of formerly Roman territory