The biggest (in size) ancient architectural monument of Istanbul (and nearly impossible to visit it all) is the ruins of the walls of Constantinople. Majestic example of defensive architecture:
My short glimse to this remain of the Roman era.
Any other city in the ancient world didn't had such giant defensive walls. Wiki shortly says: " Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century."These walls saved the Constinople for more than 1000 years until the development of military technology enabled to penetrate even such unpenetratable walls. And bigger part of these walls survived until modern days.
The size of the walls are well representing by the next picture: while the old Constantinople occupied roughly the area marked in red the walls protected the area shown in picture:
Parts of the walls could be found in the heart of Old town Istanbul:
But the most interesting parts of the walls could be seen on the outer flank: relatively far away from the old town. Of course I was simply unable to make a full round up way along the walls:
Long long way:
Honor to the ancient builders:
Parts of the wall are under restoration:
Fountain and wall:
Modern roads and old walls: