Complex sentence: Although the Corfu city castle stood firm, the rest of Corfu was destroyed and the civilian population outside the castles was defenceless and suffered heavy casualties, while homes, churches and public buildings were burned in the city suburbs.
Split into simpler sentences: Although the Corfu city castle stood firm, the rest of Corfu was destroyed and the civilian population outside the castles was defenseless and suffered heavy casualties. Homes, churches and public buildings were burned in the city suburbs.