fortified, were unoccupied with the exception of a small area in the immediate environs of Kursk. The combined depth of the three main defensive zones was about . The six defensive belts on either side of Kursk were deep. If the Germans managed to break through these defences they would still be confronted by additional defensive belts to the east, manned by the Steppe Front. These brought the total depth of the defences to nearly . The Voronezh and Central Fronts dug and of trenches respectively, laid out in criss-cross pattern for ease of movement. The Soviets built more than 686 bridges and about of roads in the salient. Red Army combat engineers laid 503,663 anti-tank mines and 439,348 anti-personnel mines, with the highest concentration in the first main defensive belt. The minefields at Kursk achieved densities of 1,700 anti-personnel and 1,500 anti-tank mines per kilometre, about four times the density used in the defence of Moscow. For example, the 6th Guards Army of the Voronezh Front, was spread out over nearly of front and was protected by 69,688 anti-tank and 64,430 anti-personnel mines in its first defensive belt with a further 20,200 anti-tank and 9,097 anti-personnel mines in its second defensive belt. Furthermore, mobile obstacle detachments were tasked with laying more mines directly in the path of advancing enemy armoured formations. These units, consisting of two platoons of combat engineers with mines at division level and one company of combat engineers normally equipped with 500–700 mines at corps level, functioned as anti-tank reserves at every level of command. In a letter dated 8 April, Zhukov warned that the Germans would attack the salient with a strong armoured force: Nearly all artillery, including howitzers, guns, anti-aircraft and rockets, were tasked with anti-tank defence. Dug-in tanks and self-propelled guns further strengthened the anti-tank defences. Anti-tank forces were incorporated into every level of command, mostly as anti-tank strong points with the majority concentrated on likely attack routes and the remainder amply spread out elsewhere. Each anti-tank strong-point typically consisted of four to six anti-tank guns, six to nine anti-tank rifles, and five to seven heavy and light machine guns. They were supported by mobile obstacle detachments as well as by infantry with automatic firearms. Independent tank and self-propelled gun brigades and regiments were tasked with cooperating with the infantry during counterattacks. Soviet preparations also included increased activity of Soviet partisans, who attacked