Complex sentence: Most of the city's churches, cathedrals, and other monuments did not survive, while the city center, dating from medieval times, was totally destroyed in a firestorm in which some 5,000 people perished.
Split into simpler sentences: Most of the city's churches, cathedrals, and other monuments did not survive. The city center, which dated from medieval times, was totally destroyed in a firestorm in which some 5,000 people perished.