The Theodosian walls are among the oldest continuous human structures in the city — built in 413, the same year Augustine started City of God, and they kept holding while emperors died and dynasties changed. Yedikule is the southern anchor: a seven-towered fortress Mehmed II built in 1458, incorporating the Byzantine Golden Gate, where every triumphant emperor for a millennium entered the city.
Four kilometres along the stones, a hundred minutes at a reading pace. Start at Yedikule Marmaray; the fortress is three minutes east. Walk the ramparts of Yedikule first — the two marble pylons of the Golden Gate are still inside, blackened from the Ottoman fortification but unmistakable.
Then north along the walls. The path runs between the inner and outer curtain; in spring, poppies grow in the dry moat. Belgrade Gate is the first major gap. Keep walking. Each tower is its own ruin.
End at Topkapı land gate — the cannon gate Mehmed's forces breached on 29 May 1453. The T1 tram is right there.