Complex sentence: The channel in which Willaumez chose to position his fleet formed a strong defensive position: an assailant had to cross the open Basque Roads and advance past the long and dangerous Boyart Shoal hidden just below the surface.
Split into simpler sentences: The channel in which Willaumez chose to position his fleet formed a strong defensive position. An assailant had to cross the open Basque Roads and advance past the long and dangerous Boyart Shoal hidden just below the surface.