Why did Constantinople's walls hold attackers off for 1000 years?

Answer: Triple-layer wall plus moat plus impossible terrain

Triple-layer wall plus moat plus impossible terrain ✓ — Correct! The Theodosian Walls had a moat, then an outer wall, then a tall inner wall — three lethal layers in 60 m of depth. Plus the city sits on a peninsula with sea on two sides and a chain across the Golden Horn harbor. Attackers had to crack three walls AND solve the navy problem. The walls finally fell in 1453 only because Ottoman cannons (60-ton bombards) could shatter the inner wall at distance — the same materials-physics problem that ended European wall warfare.

Walls were made of an unbreakable Roman concrete — Wrong. The walls were standard Roman/Byzantine brick-and-mortar — strong but not magic. The defensive power came from the LAYERED design: each layer forced attackers to fight three battles, not one.

Attackers always ran out of food before the walls fell — Wrong. Several sieges saw attackers well-supplied (Avars 626, Arabs 717, Bulgars 813). Food helped some defenders, but the walls themselves did the work — the geometry was the weapon.