Complex sentence: Whilst they were still finding their range a lucky shot hit one of the chains holding up the drawbridge and a small troop of alert roundheads were able to swiftly gain entry, putting an end to the siege almost before it had started.
Split into simpler sentences: Whilst they were still finding their range a lucky shot hit one of the chains holding up the drawbridge and a small troop of alert roundheads were able to swiftly gain entry. Putting an end to the siege almost before it had started.