Complex sentence: It is said that R. L. Stevenson based his map of "Treasure Island" on the shape of Fidra Fidra is also attended by islets called North Dog and South Dog and Castle Tarbet is an islet to the south separated from the main bulk at high tides.
Split into simpler sentences: It is said that R. L. Stevenson based his map of "Treasure Island" on the shape of Fidra. The island is also attended by companion islets called North Dog and South Dog and Castle Tarbet is another islet to the south separated from the main bulk at high tides.