Complex sentence: It had escaped major damage during most of World War II, but was heavily damaged by "Blockbuster bombs" dropped by the British Royal Air Force on April 14,1945, three weeks before the end of the war.
Split into simpler sentences: It had escaped major damage during most of World War II, but was heavily damaged by "Blockbuster bombs" dropped by the British Royal Air Force on 14. April 1945, three weeks before the end of the war.