What achievements of Byzantium in the early Middle Ages do you consider to be important?

We are talking about the Eastern Roman Empire in the period between the end of the 5th and the beginning of the 11th century, the achievements were as follows:

Military: Greek fire, invented in the 7th century and used to destroy enemy ships.
Legal. The Codex of Justinian can be considered the best example; it was created in the 6th century.
Territorial. By the middle of the 6th century AD, Byzantium managed to take control of part of the former Western Roman Empire, up to Gibraltar, for a short period.
Architecture.