Complex sentence: The castle, now a ruin, was built in the 12th century and added four towers and a perimeter wall in the 1350s; it was excavated in the 19th century (from 1835) and is now restored.
Split into simpler sentences: The castle, now a ruin, was built in the 12th century. Four towers and a perimeter wall were added in the 1350s, It was excavated in the 19th century (from 1835) and is now restored.