Complex sentence: The castle is surrounded by a water-filled, 16-sided moat, across, accessed over a 20th-century bridge into the gatehouse, an 18th-century design based on a simpler 16th-century original.
Split into simpler sentences: The castle is surrounded by a water-filled, sixteen-sided moat, across, accessed over a 20th-century bridge. The bridge crosses into the gatehouse, an 18th-century design based on a simpler 16th-century original.