Complex sentence: Peter Jackson, "The Mongols and the West", p.315 Mongols besieged the castle in Gilan for so long, that epidemic and lack of food supply forced Gilans to submit to them.
Split into simpler sentences: Peter Jackson, "The Mongols and the West", p.315 Mongols besieged the castle in Gilan so long. Epidemic and lack of food supply forced Gilans to submit to them.