Complex sentence: The Hospitallers made Krak des Chevaliers a centre of administration for their new property, undertaking work at the castle that would make it one of the most elaborate Crusader fortifications in the Levant.
Split into simpler sentences: The Hospitallers made Krak des Chevaliers a centre of administration for their new property. The work they undertook at the castle would make it one of the most elaborate Crusader fortifications in the Levant.