Which Byzantine emperor codified Roman law in the 6th century?

Justinian, whose commission spent years condensing a thousand years of accumulated Roman legal opinion - contradictory, buried in commentaries, much of it obsolete - into an organised code. The Corpus Juris Civilis is the foundation of the civil law tradition that most of the world's legal systems now use.

It was nearly lost. The work was in Latin at a moment when the eastern empire was shifting to Greek, and it fell out of use in the West with the collapse of Roman administration - then a manuscript of the Digest turned up in Italy in the eleventh century and was taken up at Bologna, which is essentially why that university exists. The revival of Roman law and the birth of the European university are the same event.

Justinian's reign was also the one that nearly ended in the Hippodrome. The Nika riots in 532 burned much of Constantinople and proclaimed a rival emperor; he was preparing to flee when Theodora refused to go, arguing that royal purple makes a fine burial shroud. He stayed, his generals penned the crowd into the Hippodrome, and something like thirty thousand people were killed. He then rebuilt the burnt city, including Hagia Sophia, whose dome was the largest in the world for nearly a thousand years.