 These divisions guarded an 18 @-@ mile ( 29 km ) perimeter which had an almost continuous antitank ditch , extensive barbed wire fence , and a double row of strong points . The strong points were situated approximately 800 yards ( 730 m ) apart . Each had its own antitank ditch , concealed by thin boards . They were each armed with one or two 47 mm antitank guns and two to four machine guns . The weapons were fired from concrete sided pits connected by trenches to a deep underground concrete bunker which offered protection from artillery fire . However , the trenches had no fire steps and the weapons pits lacked overhead cover . Each post was occupied by a platoon or company . The inner row of posts were similar , except that they lacked the antitank ditches . The posts were numbered sequentially from south to north , with the outer posts bearing odd numbers and the inner ones even numbers . The actual numbers were known to the Australians from the markings on maps captured at Sidi Barrani and were also displayed on the posts themselves . In the southern corner was a third line of posts , known as the Switch Line . There were six defensive minefields , and a scattering of mines in front of some other posts . The major tactical defect of this defensive system was that if the enemy broke through , the posts could be picked off individually from the front or rear . 
