Byzantine doesn't mean harsh or strict, it means complicated and labyrinthine. Perhaps you are thinking of Draconian?

OP, you might be interested in these earlier posts

Byzantine

• How did the adjective "Byzantine" come to have such a negative connotation?

• Just how Byzantine were Byzantine politics?

• thread in AMA - Byzantine Empire

• Where does the phrase "Byzantine view" come from? Why is a Byzantine view a bad thing?

Draconian

• How draconian was the law system in ancient athens?

Byzantium had laws that derived directly from the Roman Empire. It was also a medieval monarchy and an empire, with all the implications of one. All in all, I'd say that when it comes to insane harshness or routhlessness, byzantine laws fade in comparison to contemporary western european or islamic laws.