Defense in Depth, Medieval Style

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.

Tags: defense, history of security, physical security

Posted on April 15, 2026 at 6:47 AM • 9 Comments

Comments

AlexT • April 15, 2026 7:03 AM

Eventually defeated 🙄

Winter • April 15, 2026 7:18 AM

@AlexT

Eventually defeated 🙄

Literally only defeated after 1000 years. And then they were defeated at great costs by using guns which were invented long after it was designed and build.

I would sign up to defenses that will keep up for 1000 years.

Clive Robinson • April 15, 2026 9:48 AM

@ Winter, AlexT, ALL,

Sun Tzu observed [1], that is two simple rules,

1, Never attack the enemy when they have the high ground and supplies.

2, Never get in the enemies way when they are descending from the high ground. Let them waste energy and supplies by doing so.

Having the high ground is what these defences are in effect all about, as well as making an attackers only route of attack harder than even attacking uphill.

Modern military thinking has just reworded these ideas, and defined new battle spaces.

Hence we’ve got in modern battle space terms,

1, High Ground.
2, Air superiority.
3, Space superiority.
4, Cis-Luna superiority.

As time goes on these battle spaces will enlarge.

Consider being up in the region of space where rocks of large mass float around in Solar orbit. It takes very little effort to cause them to head down toward earth via the “gravity gradient” by which time their potential energy has converted to very large kinetic energy that is greater than many nuclear weapons…

[1] A modern translation is,

“He who occupies high Yang ground :
And ensures his line of supplies
Will fight to advantage.

On precipitous terrain :
If you occupy it first,
Yo should hold the Yang heights and wait for the enemy.

If the enemy occupies it first, do not go after him,
But entice him out by retreating.”

lurker • April 15, 2026 1:39 PM

And for fantasy Liu Cixin had it circumvented in 1453 by a bubble of four dimensional space drifting past. Most reviews ignore this opening chapter “real world” illustration of a significant plot device used later in the narrative.

https://en.wikipedia.org/wiki/Death%27s_End

Jon • April 15, 2026 2:00 PM

See also Bret Deveraux on ‘Fortifications’.

https://acoup.blog/2021/12/10/collections-fortification-part-iii-castling/

Long articles in a five-part series – still def. worth reading. Part III has (near the end) details and diagrams of the Theodisian walls.

(And, as pointed out above, resisted for a thousand years until the invention of cannon that could successfully blow holes in it).

The Historical • April 16, 2026 12:30 AM

Turkish history is a lie because Turks adopted their culture from Kurds. In many historical books, Turkish people are described as barbarians.

Vles • April 16, 2026 5:04 AM

DEATH prodded Binky who took a step forward. He could clearly see the battlefield before him.
He had stopped wondering about humans’ propensity to go out with a bang, with a low growl or a whiny voice that simply said ‘mommy’. He had stopped asking about their upbringing after speaking to a famous general. Something to do with morale.
The memory of Jeanne Calment flashed before his glowing blue eyes.
DEATH didn’t sigh, he knew too much. He shifted his shoulders around and spoke:
“I’LL BE BACK.”

Rontea • April 16, 2026 9:40 AM

The fall of the Theodosian Walls in 1453 is a classic example of how security measures fail over time, not because they were poorly designed, but because the threat model evolved. Built in the early 5th century, the walls were a marvel of layered defense—moats, outer and inner walls, towers—optimized against the siege engines, infantry, and logistics of their era. For nearly a thousand years, they withstood attacks by conventional forces because the attackers operated within the parameters the walls were meant to counter.

By the mid-15th century, the Byzantine defenders faced a fundamentally new threat: large, mobile cannons capable of sustained bombardment. The Ottomans didn’t just bring more soldiers; they brought a shift in technology that invalidated the defensive assumptions of the walls. The defenders had no effective counter to concentrated artillery fire, and the walls crumbled under sustained pressure.

This is the historical analog to cryptographic algorithms or network defenses that are secure until new computation methods—quantum computing, say—render them obsolete. Security isn’t static; it’s a process of continuous adaptation. If you assume the threats of today will be the same tomorrow, the walls—digital or stone—will eventually fall.

Clive Robinson • April 16, 2026 3:38 PM

@ Rontea, ALL,

With regards,

“The defenders had no effective counter to concentrated artillery fire, and the walls crumbled under sustained pressure.”

This attack method failed a ways a ways in the middle east.

The defenders whilst under siege realised that the walls would fail because they were made of rock that shattered because it was too hard.

So they built a wooden retaining wall inside a yard or so back and filled the gap with loose soil that had been “wetted down”…

The result a “soft wall” that the cannon shot that got through the stone just sank into and did not get as far as the wooden retaining wall.

Yes even this idea eventually failed due to the attackers going back to what we would call “Weapons of Mass Destruction”. They simply catapulted dead and rotting carcasses over the wall and waited for the inevitable diseases to overwhelm the defenders…

Proving if needed that all successful methods of attack usually have a long history behind them even modern biological and chemical WMD.

From a “Security Perspective” it”s worth studying the back and forth of attack and defence over as much history as possible. Because even in the realm of the “Intangible Information universe” most attacks and defences are really just an extension of earlier “Tangible Physical universe” attacks and defences.

Often this “comes as a shock”, I can remember our host @Bruce response on being told that a camera and a photograph of a key biting profille and lock key way, gave you all the information you needed to do what “locksmiths” did the hard way with Impressioning. This was back at a time when 3D printers were quite expensive thus very rare… And I cut keys like the UK FB1 etc using “key blanks” and “needle files”. That most key cutting places would then duplicate because they nolonger had FB1 stamped on them (talk about “security theatre fail”).

Then of course a little later we all had a good laugh when the US TSA published a photo of the “special keys” that TSA luggage inspectors were issued with…

And for a short while “Hollywood Writers” used 3D printing of keys as a plot line.

Now I suspect it’s a case of “everybody knew it”… So it’s now in effect become “forgotten knowledge”, so will “work again”. Such is the way such things go especially in the ICTSec industry…

Defense in Depth, Medieval Style

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