<P> After wintering in the western coastlands of Asia Minor , the Arab army crossed into Thrace in early summer 717 and built siege lines to blockade the city , which was protected by the massive Theodosian Walls . The Arab fleet , which accompanied the land army and was meant to complete the city 's blockade by sea , was neutralized soon after its arrival by the Byzantine navy through the use of Greek fire . This allowed Constantinople to be resupplied by sea , while the Arab army was crippled by famine and disease during the unusually hard winter that followed . In spring 718 , two Arab fleets sent as reinforcements were destroyed by the Byzantines after their Christian crews defected , and an additional army sent overland through Asia Minor was ambushed and defeated . Coupled with attacks by the Bulgars on their rear , the Arabs were forced to lift the siege on 15 August 718 . On its return journey , the Arab fleet was almost completely destroyed by natural disasters and Byzantine attacks . </P>
<P> After wintering in the western coastlands of Asia Minor , the Arab army crossed into Thrace in early summer 717 and built siege lines to blockade the city , which was protected by the massive Theodosian Walls . The Arab fleet , which accompanied the land army and was meant to complete the city 's blockade by sea , was neutralized soon after its arrival by the Byzantine navy through the use of Greek fire . This allowed Constantinople to be resupplied by sea , while the Arab army was crippled by famine and disease during the unusually hard winter that followed . In spring 718 , two Arab fleets sent as reinforcements were destroyed by the Byzantines after their Christian crews defected , and an additional army sent overland through Asia Minor was ambushed and defeated . Coupled with attacks by the Bulgars on their rear , the Arabs were forced to lift the siege on 15 August 718 . On its return journey , the Arab fleet was almost completely destroyed by natural disasters and Byzantine attacks . </P>
what helped protect constantinople from invasion by land
what helped protect constantinople from invasion by land
Theodosian Walls
Theodosian Walls