Why Castles Had Moats, and What Was In Them

Why did castles have moats? To keep attackers away from the bottom of the wall. That is the real answer, and it is less fun than the version with crocodiles in it. A moat held ladders and battering rams at a distance, it stopped miners tunnelling under a tower, and it forced everyone who arrived to use one crossing that the gatehouse could cover. Castles with moats are some of the best looking buildings in Britain and Ireland, but the ditch was engineering before it was scenery. If you want the wider story of what castles were built to do, that guide sets it out.

What a moat was really for

Three jobs, in rough order of importance.

The first was distance. A siege came down to getting heavy things close to a wall: scaling ladders, rams, timber towers on wheels, men with picks. A wide ditch pushes all of that back. Even a dry one means an attacker has to climb down, cross the bottom in the open, and climb up the far side while the garrison drops things on them.

The second was mining. One of the most reliable ways to break a stone castle was to dig a tunnel under a corner tower, prop the roof with timber, then set the props alight so the tower came down. Water in the ditch makes that close to impossible, because the tunnel floods long before it reaches the footings. Wet ground around a castle also keeps the water table high, which makes any digging slow and wretched.

The third was the door. A moat turns an open perimeter into a single controlled gap. Everything the defenders had, the gate, the portcullis, the archers, could then be concentrated on that one point instead of spread thin around the whole circuit.

Wet moats, dry ditches and what survives

Far more castles had a dry ditch than a water-filled moat. Water needs a source you can dam, a stream, a spring or a river, plus ground low enough and heavy enough to hold it. Castles built on rock or a ridge got a ditch cut into the bedrock instead.

A dry ditch is not a failed moat. On a steep site it often works better, because the sides stay sharp and nothing has to be maintained. Many of the ditches you can walk through today were never meant to be wet.

What survives is usually neither, though. Ditches silt up, banks slump into them, and ploughing smooths the rest. What is left is a shallow green dip that only reads as a defence once someone points it out. That is the same reason earthwork sites look so modest, as the four main castle types explains. One piece of trivia clears up a lot of confusion here: moat and motte come from the same old French root, and it first meant the mound, not the hole beside it.

Drawbridges, gatehouses and the way in

People go looking for castles with drawbridges and are often disappointed. Almost none are original. Timber rots, iron rusts, and a working bridge is the first part of a castle to go. What you cross today is nearly always a later rebuild, sometimes a convincing one.

The designs fell into a few patterns. The best known lifted on chains that ran through slots in the front of the gatehouse to a winch in the room above, so a few people could haul up a heavy span. A second type used counterweighted beams swinging in deep slots, which took much less effort. A third pivoted on an axle part way along, so pushing one end down tipped the other into a pit inside the passage. Most crossings were mainly fixed bridge, with only the last section able to move.

A drawbridge never worked alone. Behind it came a portcullis, then heavy doors, sometimes a second portcullis, with openings in the vault above so anyone trapped in the passage could be dealt with from upstairs. The gatehouse was the strongest and most expensive part of a late medieval castle for exactly this reason.

To read it on site, look above the gate arch for a pair of vertical slots and the chain holes, then look down at the floor just inside for the pit the bridge dropped into.

The crocodile moat, and what was actually in there

There were no crocodiles. Not in Britain, not in Ireland, and not in the climate. No alligators, no sharks, no rows of spikes under the surface. Those come from films and theme parks, and they get asked about more than anything else.

What a wet moat actually held was water, silt, reeds and fish. Stocked fish were a genuine benefit, a larder that needed no feeding. Moats also took waste. Latrine chutes and kitchen drains at plenty of castles ran toward the ditch, so the water closest to the building was foul. The smell was one reason later owners wanted rid of it.

Moated castles that still have water

Bodiam Castle in East Sussex is the one almost everybody pictures, a square castle sitting in a broad rectangular lake, with an approach designed as much for show as for defence. Leeds Castle in Kent goes further, with buildings standing on islands in a lake made by damming a river. Hever, also in Kent, keeps its water and its bridge.

Caerphilly in south Wales is the most ambitious water defence of the lot. Long dams hold back lakes on two sides, so the castle reads as a fortress on a shoreline rather than a building in a ditch.

In Scotland the pickings are thinner, because so many castles sit on crag or coast. Caerlaverock near Dumfries is the clear answer, a triangular castle still ringed with water. Rothesay on Bute is the odd one out, round in plan with a wet moat you cross at the gate. Ireland has a different pattern: thousands of modest moated sites, rectangular ditched enclosures around a house, rather than great water castles.

How most moats were drained and lost

Once artillery arrived, a ditch stopped deciding anything. Castles that carried on in use turned into homes, and a home wants lawns, views and drains, not standing water under the windows. Some moats were kept as ornamental water and a few were widened into lakes. Many more were filled, levelled for gardens or taken into surrounding fields.

Health worries finished off others, since stagnant water next to a house was blamed for disease. Changes upstream did the rest. Mills, land drainage and rerouted streams cut off the supply, and a moat with no inflow becomes a damp hollow within a generation or two.

Where to go next

If you want to stand in one, use the full castle index to find the moated sites near you, then check each entry before you drive out. Our castles near pages do the same job by city if you would rather start from where you live.