Complex sentence: Two colonels, Austen and Phelps, were both killed as the 12th and 14th headed for the centre of the kilometre-long line, unaware it was the location of the redoubt, its strongest section.
Split into simpler sentences: Two colonels, Austen and Phelps, were both killed. The 12th and 14th headed for the centre of the line, unaware it was the location of the redoubt, its strongest section.