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.

The US National Reconnaissance Office has declassified information about a fleet of spy satellites operating between 1971 and 2006.

I’m actually impressed to see a declassification only two decades after decommission.