Complex sentence: There are ruins of several small early Byzantine castles along the coast between the rivers Veleka and Rezovska, which were part of the defensive system built by emperor Justinian I (r.527--565) against Avar and Slavic incursions.
Split into simpler sentences: There are ruins of several small early Byzantine castles along the coast between the rivers Veleka and Rezovska. These were part of the defensive system built by emperor Justinian I (r.527--565) against Avar and Slavic incursions.