Complex sentence: The MacLeods opened fire from the ditches and from behind walls, but were outnumbered, and being vigorously pressed, they gave way and retreated, and were pushed back to Elgin.
Split into simpler sentences: The MacLeods opened fire from the ditches and from behind walls. They were outnumbered, and being vigorously pressed, they gave way and retreated, being pushed back to Elgin.