Reading a castle from the outside in
The best way to understand castle architecture is to approach one the way an attacker would. First comes the ground, then any water or ditch, then the wall, and finally the gate. Each feature was designed to answer a specific problem. This article takes those features in order. For the long history of how designs changed, the broader overview of castle development covers the sequence; here the focus is on the parts themselves.
The site
Builders chose ground carefully. A rock outcrop, a hilltop, a river bend or a sea cliff could provide natural protection on one or more sides, saving work and making an approach difficult. Where the land was flat, builders could dig ditches or flood the surroundings. Choosing a site also meant considering water supply, since a castle without a reliable well could not hold out for long.
Walls
The curtain wall formed the main barrier. Most stone walls were built with outer faces of dressed or rough stone and a core filled with rubble and mortar. Thickness varied with the period and the castle’s importance; in many large castles it was several feet or more, enough to withstand missiles and make breaching slow.
Many walls were built with a sloping or flared base, called a batter or plinth. This helped deflect objects dropped from above in a way that sent them bouncing toward attackers, and it made undermining harder because a wider base is harder to dig under. A siege engine could still damage a wall, but thick, well-built masonry could take much punishment.
Battlements and the wall walk
Along the top ran a wall walk, a passage for guards and defenders. On the outer side was a parapet with alternating raised sections (merlons) and gaps (crenels). Defenders could shoot through the gaps and shelter behind the raised parts. In some castles, shutters or hinged boards closed the gaps. Timber galleries called hoardings could be built out over the wall to allow defenders to drop objects on attackers at the foot; these were quick to install in an emergency but could burn. In later castles, permanent stone versions, called machicolations, projected from the wall top.
Towers
Towers rose above the wall and projected outward, so that defenders could cover the length of the wall from the side. This flanking fire was one of the main advances in castle design. Early towers were often square, but round and D-shaped towers became common because they deflect missiles better, give fewer blind spots and resist undermining more effectively than sharp corners.
Towers also provided places to live and store things, and a high tower gave a good view. In many castles, individual towers could be sealed off and defended even if neighboring sections fell. The great tower or keep, discussed in other articles, was the largest and strongest of these.
Arrow loops
Narrow vertical slits in walls and towers, known as arrow loops, allowed archers to shoot while presenting a tiny target. Inside, the opening often widened so an archer could stand within the thickness of the wall and aim. Some had crosspieces to allow crossbow use. They were a good example of a compromise between protection and effectiveness: the narrower the slit, the safer the archer, but the harder it was to aim.
The gatehouse
The gate was the most likely point of attack, and the gatehouse was the strongest part of many castles. Typical elements included:
- A drawbridge or fixed bridge across the ditch, sometimes with a section that could be raised or removed.
- A portcullis, a heavy grille that could be lowered to block the passage. Usually made of wood reinforced with iron, it was raised and lowered with a winch.
- Heavy wooden doors, sometimes reinforced with iron, and a bar.
- A passage with openings in the ceiling (often called murder holes) from which defenders could drop objects or throw water to put out fires.
- Flanking towers on either side, with arrow loops covering the entrance.
- A barbican, an outwork in front of the gate, forcing attackers into a narrow, exposed approach.
A twisting or doubled entrance slowed attackers further. Lords also lived above gatehouses in some castles, so these buildings served as both defensive and residential spaces, as well as a way of showing off status.
Moats and ditches
Moats could be wet or dry. A wet moat kept siege towers and rams at a distance and made mining difficult, because tunnels flooded. A dry ditch slowed attackers and left them exposed to missiles. Many “moats” were fed by diverting a stream or river, and some were broad lakes. Not every castle had one, as it depended on the terrain, and a moat around a stone wall also posed engineering challenges. Some moats were more about display than defense, a point developed in the article on fortress and symbol.
Postern gates and escape routes
A small side door, called a postern, allowed sorties against besiegers and the quiet movement of messengers. It was a weak point and had to be small and well guarded. Some castles also had a sea gate or water gate to receive supplies.
How the parts worked together
No single feature was meant to stop an attack. Walls forced the attacker to climb or breach, towers allowed the defenders to shoot along them, the ditch kept equipment away, and the gatehouse limited the number who could enter at once. Together with a stocked storehouse and a determined garrison, these elements made a castle expensive to attack. The same logic applied whether the engine used against it was a battering ram or one of the trebuchets and catapults used from the 12th century onward, and the builders, many of them medieval stonemasons working for years, solved these problems one feature at a time.
Remember that nearly every surviving castle has been altered. Walls have been repaired, windows enlarged and upper works lost, so the version we see is a layered record of centuries of change and not a single moment.