Complex sentence: It then became one of the main Christian centres in Lower Nubia and held out against the Muslims until the 16th century, when a group of Bosnian mercenaries, part of the Ottoman army, occupied the site.
Split into simpler sentences: It then became one of the main Christian centers in Lower Nubia. The city held out against the Islam until the 16th century, when a group of Bosnian mercenaries, part of the Ottoman army, occupied the site.