Complex sentence: Access to the castle is via a causeway from St Helier at low tide, or on a "Castle Ferry", these are wading vehicles which can reach the castle regardless of tide height, weather permitting.
Split into simpler sentences: Access to the castle is via a causeway from St Helier at low tide, or on a "Castle Ferry". The ferries are wading vehicles that can reach the castle regardless of tide height, weather permitting.