Complex sentence: The forts and were sited to overlook each other for mutual support and the outer ring, with a circumference of, had in shell-proof turrets and more than and machine-guns to protect the ditches around the forts.
Split into simpler sentences: The forts and had been sited to overlook each other for mutual support and the outer ring had a circumference of. The outer forts had in shell-proof turrets and more than and machine-guns, to protect the ditches around the forts.