The impression made by the mighty Theodosian Walls on the Western Crusaders who encountered them can be seen in the 13th century Caernarfon Castle in Wales, built by Edward I of England as a royal residence, which is said to have been modelled on them. With the advent of siege cannons, however, the fortifications became obsolete, but their massive size still provided effective defence, as demonstrated during the Second Ottoman Siege in 1422. In the final siege, which led to the fall of the city to the Ottomans in 1453, the defenders, severely outnumbered, still managed to repeatedly counter Turkish attempts at undermining the walls, repulse several frontal attacks, and restore the damage from the siege cannons for almost two months. Finally, on 29 May, the decisive attack was launched, and when the Genoese general Giovanni Giustiniani was wounded and withdrew, causing a panic among the defenders, the walls were taken. After the capture of the city, Mehmed had the walls repaired in short order among other massive public works projects, and they were kept in repair during the first centuries of Ottoman rule.

Many sections were restored during the 1980s, with financial support from UNESCO, but the restoration programme has been criticised for focusing on superficial restoration and poor quality of work, which became apparent in recent earthquakes, as well as destroying historical evidence.Nonetheless, the restored sections give a fairly accurate image of the walls as they stood during Byzantine times. The wall runs through the suburbs of modern Istanbul, with a belt of parkland flanking their course. The walls are pierced at intervals by modern roads leading westwards out of the city.