Complex sentence: The castle was well supplied for a siege, but the extra mouths to feed rapidly diminished them; between 1,400 and 2,200 non-combatants were allowed inside, increasing the number of people in the castle at least fivefold.
Split into simpler sentences: The castle was well supplied for a siege, but the extra mouths to feed rapidly diminished the stores. Between 1,400 and 2,200 non-combatants were allowed inside, increasing the number of people in the castle at least fivefold.