Complex sentence: Its unique position--on an island just 600 metres from land--made it accessible at low tide to the many pilgrims to its abbey, but defensible as an incoming tide stranded, drove off, or drowned, would-be assailants.
Split into simpler sentences: Its unique position of being an island only 600 metres from land made it readily accessible at low tide to the many pilgrims to its abbey. Equally, this position made it readily defensible as an incoming tide stranded, drove off, or drowned, would-be assailants.