Complex sentence: In AD 1220, it strongly resisted the Mongol hordes and was thus laid to waste-around 20,000 Mongol soldiers surrounded the city and besieged it but a local man opened the doors of the city and let the Mongol army in.
Split into simpler sentences: In AD 1220, it strongly resisted the Mongol hordes and was thus laid to waste. Around 20,000 Mongol soldiers surrounded the city and besieged it but a local man opened the doors of the city and let the Mongol army in.