Complex sentence: The Christian kingdoms of Nubia came under pressure from the 7th century, but they resisted for several centuries, until the Kingdom of Makuria and Old Dongola collapsed by the beginning of the 14th century.
Split into simpler sentences: The Christian kingdoms of Nubia came under pressure from the 7th century, but they resisted for several centuries. The Kingdom of Makuria and Old Dongola collapsed by the beginning of the 14th century.