The Soviets had begun constructing an inner defensive line extending out 5-8km from Sevastopol on 4 July 1941, as well as planning a main defensive line extending out to 10km from the city and a forward line extending out even farther. Some barbed wire and mines were emplaced in August but the two outer lines were still incomplete when German forces began approaching the city in October 1941. The SOR was divided into four defensive sectors. The two sectors most likely to be attacked by the enemy were Defensive Sector IV, which covered the Belbek River Valley and the western coast, and Defensive Sector III, which covered the hilly area between Kamyschly and Mekenzyya Village; both of these sectors had a front-line length of 8.5km. Defensive Sector II covered the Chernaya River Valley and the approaches to the vital Sapun Heights; this sector was the widest with a front-line length of 12km. Defensive Sector I, which was only 7.5km long, was extremely well fortified and covered the mountainous terrain between Rose Hill and Balaklava. Each sector was commanded by one of the division commanders, but the naval coastal batteries in Defensive Sectors I and IV remained under the direction of General-Major Petr Morgunov, commander of Sevastopol's coastal defences.
In each defensive sector, Soviet troops had constructed trenches, bunkers, minefields and wire obstacles to supplement the existing coastal defences. Throughout the siege the Soviets continued to improve these fortifications and by May 1942 they had become very formidable. Oktyabrsky and Petrov intended to use their dense obstacle belt to wear down any attacking German infantry in an attritional battle, a formula that they had employed with great success against the Romanians at Odessa in 1941. As long as the Black Sea Fleet could keep delivering infantry replacements to keep the ground defence viable, this plan had good chances of success. In time, the Germans would weary and stop, as they had in November and December 1941. Soviet strategy in the Crimea in 1942 merely required Sevastopol to hold out, with the main burden for operational success resting with the forces in the Kerch Peninsula.
However, the Stavka's plans for the Crimea were undermined by a dangerous underestimation of German capabilities in the area and an exaggerated sense of Sevastopol's impregnability. The Stavka did not expect any major Axis reinforcements for the Crimea in spring 1942 and were particularly dismissive of the effects of Luftwaffe air superiority over Sevastopol. Too much Soviet effort went into sustaining the Kerch lodgement with the result that Sevastopol's defenders did not have adequate ammunition or reserves for a protracted battle. Oktyabrsky and Petrov based their defensive plans on two ultimately false assumptions - that any renewed German assault would last for no more than ten days and that the garrison could count upon a Soviet offensive from the Kerch Peninsula to draw off von Manstein's reserves.