from the 11th century (ditches much older)
Moat
A ditch, dry or filled with water, that keeps attackers, rams and towers away from the wall.
The simplest defence of all is a hole in the ground. A deep ditch in front of the wall stops a rolling siege tower or a battering ram before it can reach the stone, and anyone climbing out of it is an easy target from above.
Many moats were dry. Where a river or spring allowed, builders flooded them, which also made digging a mine under the wall almost impossible. The earth thrown out of the ditch was often piled up to make a bank or a mound.
To cross a moat, besiegers had to fill a stretch of it with earth, brushwood and rubble while under fire, a slow and costly job that gave the defenders time.
To see one: Bodiam Castle, England; Caerlaverock Castle, Scotland
from the 12th century
Drawbridge
A bridge over the ditch that could be raised, swung or pulled back to leave a gap in front of the gate.
A fixed bridge is a gift to an attacker, so the last span before the gate was made to move. Early versions simply slid back; later ones pivoted like a seesaw, with the inner end dropping into a pit as the outer end rose against the gate.
Raised, the bridge left an open drop in front of the gateway and added one more layer of timber over the door. Many later gatehouses kept the slots for the lifting beams or chains in their walls, even where the bridge itself has long gone.
Drawbridges were heavy and slow, so they were usually kept down by day and raised at night or when danger came close.
To see one: Pevensey Castle, England (drawbridge pit); Château de Vincennes, France
from the late 12th century
Barbican
An outwork in front of the gate that forces attackers into a narrow, watched approach.
The gate was always the weakest point, so builders pushed a second, smaller defence out in front of it. A barbican might be a walled passage, a pair of towers or an earth-and-timber enclosure, but its job was the same: make the enemy fight twice before reaching the real door.
Inside a barbican the approach often turned a corner, so a ram could not get a straight run and attackers had to show their unshielded sides to archers on the walls.
To see one: Warwick Castle, England; Lewes Castle, England
from the 11th century; twin-towered gatehouses from c. 1190
Gatehouse
A fortified building around the entrance, packed with doors, grilles and firing points.
Early castles had a simple tower over the gate. By the late 12th and 13th centuries the gatehouse had become the strongest part of the castle: a long passage between two round towers, closed by several doors and portcullises, with murder-holes in the vault and arrow-slits in the side walls.
The largest gatehouses, such as those built for Edward I in Wales, were small castles in themselves, with halls and chambers above the passage. They could be held on their own even if the rest of the castle fell.
To see one: Harlech Castle, Wales; Caerphilly Castle, Wales
known in Roman times; common in castles from the 12th century
Portcullis
A heavy grille of timber shod with iron that drops down grooves to block the gate passage.
A portcullis hung in grooves cut into the walls of the gate passage and was lifted by a winch in the room above. Released, it fell under its own weight, which made it the fastest way to shut a gate when attackers were already running for it.
Gatehouses often had two or more, one at each end of the passage. Shut both, and any attackers caught between them were trapped beneath the murder-holes. Because it was a grille rather than a solid door, defenders could still shoot and stab through it.
To see one: Bloody Tower, Tower of London, England; Bodiam Castle, England
from the 12th century
Murder-holes
Openings in the vault of a gate passage through which defenders could drop missiles or water.
In the ceiling of many gate passages are square or round holes. Defenders in the room above could drop stones on anyone forcing the gate, or shoot down through them.
Popular stories of boiling oil are mostly exaggerated: oil was expensive. Hot water, sand and quicklime are better attested, and some historians think many of these holes were mainly for pouring water to put out fires set against the wooden doors.
To see one: Bodiam Castle, England; Beaumaris Castle, Wales
in Europe mainly from the late 13th century
Machicolations
Stone galleries jutting out on corbels from the wall top, with openings in the floor to drop missiles straight down.
From a flat wall-walk, a defender cannot see the foot of his own wall without leaning out over it. Machicolations solve this. The parapet is pushed outwards on a row of stone brackets (corbels), and the gaps between the brackets are left open to the ground below.
They are the stone, permanent version of the wooden hoardings that came before them, and they could not be set on fire. In the late Middle Ages they became so fashionable that some were built mainly for show.
To see one: Château de Vincennes, France; Raglan Castle, Wales
12th–14th centuries
Hoardings
Temporary wooden galleries built out from the top of a wall in wartime, to cover its foot.
When a siege was expected, carpenters slid beams through square holes left just below the parapet and built a covered wooden balcony on them. Through gaps in its floor, defenders could hit sappers and rams at the base of the wall.
Hoardings were cheap and quick but they burned and could be smashed by stone-throwers, which is one reason they were replaced by stone machicolations. The rows of beam holes, called putlog holes or hoarding holes, often survive where the woodwork does not.
To see one: Carcassonne, France (19th-century reconstruction); Château de Laval, France
ancient; on almost every castle
Crenellations
The notched top of a wall: solid merlons to hide behind and open embrasures to shoot through.
The familiar tooth-like outline of a castle wall gave defenders both cover and a line of fire. The raised parts are merlons; the gaps between them are embrasures, or crenels. Merlons sometimes had their own arrow-slits, and wooden shutters could close the gaps.
In medieval England a lord needed a royal 'licence to crenellate' to fortify his house. Historians now see these licences as much about status as about defence.
To see one: Conwy Castle, Wales; Carcassonne, France
from the late 12th century
Arrow-slits
Narrow openings in a wall that let an archer shoot out while barely exposing himself.
From the outside an arrow-slit looks like a thin crack. Inside, the wall is cut back into a wide alcove, so the archer can stand in shelter and aim to either side. Crossbows suited them especially well, because they could be loaded out of sight.
Slits came in many shapes: plain, cross-shaped for a wider view, with round 'oillets' at the ends, or grouped so several archers could cover one approach. After about 1400 many were widened at the bottom into gunloops.
To see one: Caernarfon Castle, Wales; Goodrich Castle, England
in stone from the 11th century
Curtain wall
The main wall that encloses the castle and links its towers.
The curtain is the long stretch of wall between towers, like a curtain hung between posts. It was usually two or three metres thick, with a wall-walk on top behind a crenellated parapet.
A plain curtain is vulnerable to mining and to battering at its foot, which is why builders added projecting towers along it. In the 13th century the best castles had two curtains, one inside the other.
To see one: Conwy Castle and town walls, Wales; Pembroke Castle, Wales
from the late 12th century (Roman forerunners)
Mural tower
A tower set into the curtain wall and projecting from it, so defenders can shoot along the wall face.
A tower that sticks out from the wall lets archers inside fire sideways, along the face of the curtain, at anyone trying to climb or dig at it. This is called flanking fire, and it is the key idea of castle design from about 1200 onwards.
Early towers were square, but corners are easy to undermine and leave blind spots. Round and D-shaped towers became the norm in the 13th century. Many towers were open at the back, so an enemy who took one could not use it against the rest of the castle.
To see one: Conwy Castle, Wales; Framlingham Castle, England
c. 1070–1200 at its height
Keep
The great tower: the lord's residence and the last stronghold if the outer defences fell.
The first stone castles of the Normans were built around a huge rectangular tower, the keep or donjon. Its walls could be four metres thick, the door was raised to first-floor level and reached by an outside stair, and it held halls, chambers, a chapel and often its own well.
Keeps were symbols of power as much as fortresses. Their weakness was their square corners, which could be mined. From about 1200, builders put more effort into strong outer walls and towers, and the keep lost its central role.
To see one: White Tower, Tower of London, England; Rochester Castle, England
from the 11th century
Bailey
The enclosed courtyard inside the walls, where the castle's daily life went on.
Inside the curtain lay the bailey, also called a ward. It held the buildings a castle needed to run: hall, kitchens, stables, workshops, stores, a chapel and lodgings for the garrison.
Many castles had more than one bailey, each walled off from the next. An attacker who broke into the outer bailey still faced another wall, and the defenders could fall back step by step.
To see one: Windsor Castle, England; Pickering Castle, England
from the 12th century
Postern
A small secondary door, often hidden, for messengers, supplies or a surprise sortie.
Besides the great gate, most castles had one or more small doors, usually in a hard-to-reach corner. A postern let a messenger slip out, food be brought in by a back route, or a group of defenders sally out to burn the enemy's engines.
Because a postern could also let an enemy in, it was narrow, easily blocked and often high above the ground, reached by a ladder or a steep path.
To see one: Harlech Castle, Wales (the 'Way from the Sea'); Beaumaris Castle, Wales
throughout
Well
A secure water supply inside the walls, without which no castle could hold out for long.
A garrison can ration food for months, but without water it lasts only days. Builders went to great lengths to dig wells inside the walls, sometimes cutting through solid rock to well over 100 metres. Keeps often had their own well shaft rising through the floors.
Rainwater cisterns were used where wells were impossible. Several sieges ended not by force but because the water ran out or was fouled.
To see one: Beeston Castle, England; Dover Castle, England
from the 12th century
Batter
A sloping thickened base to a wall or tower, against mining and battering.
Many castle walls flare outwards at the bottom, like a skirt. This sloping base, also called a talus or plinth, adds a mass of masonry exactly where sappers and rams attack, and keeps siege towers and ladders a little further from the wall.
It had another use. Stones dropped from the wall top hit the slope and bounced outwards, unpredictably, into the attackers below.
To see one: Krak des Chevaliers, Syria; Goodrich Castle, England (spurred tower bases)