This article on the walls of Constantinople is fascinating.

The system comprised four defensive lines arranged in formidable layers:

• The brick-lined ditch, divided by bulkheads and often flooded, 15­-20 meters wide and up to 7 meters deep.
• A low breastwork, about 2 meters high, enabling defenders to fire freely from behind.
• The outer wall, 8 meters tall and 2.8 meters thick, with 82 projecting towers.
• The main wall—a towering 12 meters high and 5 meters thick—with 96 massive towers offset from those of the outer wall for maximum coverage.

Behind the walls lay broad terraces: the parateichion, 18 meters wide, ideal for repelling enemies who crossed the moat, and the peribolos, 15–­20 meters wide between the inner and outer walls. From the moat’s bottom to the highest tower top, the defences reached nearly 30 meters—a nearly unscalable barrier of stone and ingenuity.