When you draw a castle, you draw a moat. It’s the rule. A blue squiggle around the bottom with maybe a shark fin sticking out.
But in reality, moats were the unsung heroes of medieval defense. They were cheap, effective, and absolutely disgusting. Forget the Hollywood myths about alligators and piranhas (those didn’t exist in medieval Europe). The real horror of a moat was far worse—and far more ingenious.
Here is everything you never wanted to know about the castle ditch.
1. Not All Moats Were Wet
Myth: Every moat was a shimmering lake reflecting the castle walls. Fact: Most moats were dry ditches.
Why? Water is genuinely hard to manage. It requires a constant, reliable source—a river, spring, or high water table. It breeds mosquitoes and disease. In winter, it freezes solid, creating a natural bridge for attacking troops to walk across. It also makes the outer face of the wall perpetually damp, accelerating the decay of mortar.
A dry moat, or fosse, avoided every one of these problems. It was a steep-sided trench, typically 15–30 feet deep and 20–40 feet wide, often lined with sharpened vertical stakes (fraises) at the bottom.
If you fell in, you broke your legs on the stakes. If you tried to climb the far wall, you were a slow-moving target for archers on the battlements above. If you tried to bridge it, the span required was enormous and came under sustained fire during the attempt. Simple. Brutal. Effective.
The famous water moat—like the one surrounding Bodiam Castle in East Sussex—was actually a later development, more common in the 14th and 15th centuries when castle design had evolved and lords could afford the greater engineering cost of maintaining a water feature.
2. The Real Purpose: Anti-Mining Technology
Most visitors assume moats were primarily designed to stop infantry assault. They were not. The biggest existential threat to a stone castle was a sapper.
Sappers were specialist engineers—among the most valued professionals in any medieval army. Their job was to dig tunnels beneath castle walls. They would shore up the tunnel with wooden props, pack the end space with pig fat and kindling, then set it alight. As the props burned and collapsed, the tunnel caved in, and the section of wall above it collapsed into the void.
This technique, called undermining, brought down dozens of major castles throughout the medieval period. The collapse of the south tower at Rochester Castle in 1215—achieved by King John’s sappers using the fat of forty pigs—is one of the most documented examples.
Enter the Moat: Water kills a mining operation immediately. The tunnel floods, and everyone in it drowns. Even a dry moat forced sappers to dig much deeper than usual—often to the water table or to bedrock—before they could begin their lateral tunneling. This vastly increased the time, cost, and risk of the operation.
Castle builders understood this explicitly. Many castles built in areas with high water tables were positioned specifically so that the water table was near the surface—turning the ground itself into a defensive asset. The moat was the visible expression of a water table that existed to defeat miners.
3. The Siege Engine Stopper
Siege towers were the other great threat. These were enormous wooden structures on wheels—effectively mobile assault platforms—that were pushed up to the wall, allowing soldiers to drop a drawbridge onto the battlements and pour across.
But siege towers required firm, level ground. They were top-heavy and weighed dozens of tons. Soft, waterlogged ground made them sink and tip. And they absolutely could not cross a moat without engineering preparation.
To get a siege tower to the wall, attackers first had to fill the moat. This was called making a causeway or sapping to the wall. It required thousands of bundles of brushwood (fascines), stones, earth, and timber—all carried forward under intense fire from defenders. The work was catastrophically dangerous and could take weeks.
Even then, the filled section might be narrow, forcing the tower along a restricted line where defenders could concentrate all their fire. The moat transformed a simple assault into a complex engineering operation, buying defenders days or weeks of additional time.
4. The “Gong Farmer” and the Smell
Now for the gross part. And this part is non-negotiable: it must be included in any honest account of castle moats.
In many castles, the toilets—called garderobes—were small chambers built into the thickness of the outer wall, with a shaft running down to empty directly into the moat. Not into a pipe. Not into a treatment system. Directly into the water below.
Yes. The shimmering blue moat was often an open sewer.
This had several consequences:
Biological Warfare: Falling into the moat meant almost certain infection. Even a small wound sustained in the water—a scratch from the stakes, a graze from an arrow—brought contact with a soup of pathogens. The moat was, inadvertently or deliberately, a biological hazard zone.
Deterrence through Disgust: The smell of an active castle moat in summer was reportedly detectable from a considerable distance. Medieval accounts describe the stench of siege camps, but the moat itself contributed. Attacking troops who had to wade through it to assault the wall faced a thoroughly unpleasant experience.
The Gong Farmer: Someone had to clean it. This was the job of the Gong Farmer or Nightsoil Man. By law in England, this work had to be done at night—“so as not to offend the sight of respectable persons during the day.” The Gong Farmer descended into the moat (or the castle’s internal cesspits) and shoveled the accumulated waste into barrels, which were then carted away to sell as agricultural fertilizer.
They were, by the standards of the time, reasonably well-paid. They lived in a dedicated part of town, because no one wished to be near them. They were a recognized craft guild. And they performed a function that the entire castle depended on for basic sanitation.
It was an honest living, if you had absolutely no sense of smell.
5. The Engineering: How They Built Them
Building a water moat was a significant feat of medieval engineering, particularly when no natural water source was available.
Dammed and Diverted Rivers: At Leeds Castle in Kent, the River Len was dammed and diverted to create and maintain the moat—an operation that required earthworks on a considerable scale and ongoing maintenance to keep the inlet and outlet sluices functional.
Clay Puddling: The bed and sides of a water moat had to be made waterproof. This was achieved by puddling—systematically working wet clay by foot (literally trampling it) until all air pockets were expelled and it formed an impermeable lining. This technique, virtually identical to methods used in canal construction four centuries later, required significant labor and skill.
Sluice Gates: Every functional water moat needed at least one inlet and one outlet with a controllable sluice gate. This allowed defenders to control the water level—raising it to flood attackers, lowering it for maintenance, or manipulating it to deny sappers knowledge of the water table depth.
Drawbridges and Turning Bridges: The mechanism for crossing the moat was as important as the moat itself. A simple drawbridge was raised by chains attached to the outer edge, rotating up against the gatehouse entrance. More sophisticated designs used a turning bridge (like a see-saw)—the outer half raised while the inner half descended into a pit, making the mechanism self-balancing and faster to operate.
6. Fish Farming?
It wasn’t all poop and death. In peacetime, clean moats were stocked with fish—carp, pike, tench, perch, and eel.
For a Catholic Europe that abstained from meat on Fridays, the eves of feast days, and throughout Lent (totaling around 150 days per year), a well-stocked moat was a significant source of protein. For a garrison of 50 soldiers, the moat could provide several hundred pounds of fish per year.
Some castles also kept swans on their moats. Swans served multiple purposes: they looked magnificent, they were reserved by law for the Crown (making them a status symbol), their feathers provided premium fletching for arrows, and in extremis they were edible. Swan’s meat was considered a delicacy at royal banquets.
The Moat’s Legacy
The castle moat effectively disappeared with the arrival of effective artillery. A cannon could fire at range—far enough to stay outside the moat’s defensive perimeter entirely. Thick stone walls that could absorb trebuchet shot shattered under the focused impact of cannonballs. The entire philosophy of medieval fortification became obsolete within a few decades.
But the moat lived on, transformed. By the 16th and 17th centuries, it had become a decorative feature—a reflecting pool that doubled the visual grandeur of the manor house or palace it surrounded. Hampton Court, Versailles, and dozens of English country houses retained moats not for defense but for beauty.
And every child drawing a castle still draws one—a blue squiggle that contains, unknowingly, centuries of engineering, hygiene, fish, and grim ingenuity.
Visit the best moats:
- Bodiam Castle, East Sussex: The textbook English water moat. A near-perfect reflection on still mornings.
- Leeds Castle, Kent: Arguably England’s most beautiful castle, surrounded by a broad lake formed from the diverted Len.
- Chepstow Castle, Monmouthshire: A formidable dry ditch cut from living rock, showing you exactly what the original moat concept looked like.
- Caerphilly Castle, Wales: The most sophisticated water defense in Britain—a complex of dams, lakes, and islands that covers 30 acres.