Machicolations are built to allow a garrison to attack besiegers who manage to make it past longer-range defences to the base of the castle walls. These structures are a more advanced form of earlier hoardings, which were constructed out of wood. Sometime in the 12th century elements of these sheds, which cantilevered beyond castle walls, began to be replaced by stone. Machicolations project from the battlements beyond the face of the wall and create holes through which a castle’s defenders could shoot down arrows or crossbow bolts, throw hard objects such as stones, or dump scathing liquids such as boiling water or quicklime.

Crac des Chevaliers’ machicolations offer a striking example of the cultural diffusion of military architecture. The castle boasts two distinct types of these structures: groups of three machicolations, believed to be distinctly Crusader, and continuous machicolations, which are thought to have been of Muslim origin. Though the exact builders remain a mystery, explanations for this defensive duality include the possibility that knights imitated Eastern engineering, or that the same group of masons were working for both Christian and Muslim factions during this period.

A key part of the classic 10th century motte and bailey layout, the keep, or donjon, was one of the earliest elements of a castle. As building techniques evolved, it was also the first feature to be constructed out of stone. The keep was the last line of defence – the place that a garrison would retreat to when all else was lost to make a desperate final stand. The word ‘donjon’ is believed to have derived from the Latin word ‘dominus’, meaning lord or master. In addition to its defensive function, a castle’s keep was often home to a king or castellan. The keep underwent a significant change in the 12th century when square configurations gave way to more rounded forms.

At Chateau Gaillard, Richard the Lionheart’s donjon is a shape of its own. Its exterior walls are sloped outward. At the front they join and project forward at a sharp angle. This unique form makes it more resistant to projectiles. On the opposite side, the keep backs onto a sheer cliff, making any approach from this side virtually impossible. Inside, the last line of defence is a mere eight metres in diameter. The current point of entry is believed to date from a later period, as the original door would have almost certainly been positioned above ground and reached by a ladder or stairway. With no evidence of a fireplace, well, or latrine, it appears that this particular keep was built primarily for defence.

Defensive towers come in several different forms. In castle building, the first of these structures appeared as part of the wooden palisades that fortified motte and bailey castles. Some towers are integrated into the walls. Others are structurally independent and flanked by stretches of curtain. Certain designs project out to make it easier for archers to cover adjacent stretches of wall, others have an open back so the tower can be readily supplied via crane from below, or targeted from behind if it falls to the enemy. Some towers even have a thicker, sloped base to make undermining more difficult.

Dover Castle reveals another striking difference in tower design – geometry. Originally, castle towers were predominately square. Unfortunately, this configuration leaves the structure vulnerable in areas that archers can’t cover. In the 12th century, medieval engineers begin to design circular and semicircular towers to solve this problem. Because it was built over several decades, Dover boasts square and circular towers – the former being attributed to construction under Henry II, the latter believed to have been built several years later by his son John. The castle also has polygonal towers, including the Avranches Tower, which was specifically designed to maximize the garrison’s firepower.

The arrow slit (or arrow loop) first appears in historical accounts describing the siege of Syracuse in the 2nd century B.C. Integrated into a castle’s design, they are engineered so archers can fire at the enemy under protection of a wall or merlon. Though these defensive features were seen in Greek and Roman fortifications, they does not seem to have been built into medieval European castles until the late 12th century. But they caught on fast. By the 13th century, arrow slits were standard issue for most fortifications.

Not surprisingly, Edward’s Welsh castles incorporated this military engineering ubiquitously. At Harlech, guardrooms at the ground floor on either side of the gatehouse boast these deadly features. Conwy’s arrow slits are lined with distinct red sandstone, which is harder than the dark blue grey sandstone used to build most of the castle and therefore better suited for this type of key structural stonework. And at Caernarfon, arrow slits are uniquely arranged to exploit the power of archery in a lethal way. At Conwy and Caernarfon, merlons are also slotted so that garrisons could fire from above without being exposed. The town walls were also equipped with arrow slits – the roofless, open-backed towers that line these defences were designed to guard English settlements in the wilds of Wales.

A castle’s entrance can be the weakest point of its defence, so special attention was generally paid to fortifying these key features. In fact, much of the military technology used to protect entry points dates back as early as the 3rd century B.C. In castle building, the first towers ever raised were constructed to provide protection for gatehouses. These were also one of the first features to make the transition from being built out of wood to being made out of stone. Castle entrances generally incorporate several defensive techniques, including fortified doorways, drawbridges, and portcullises.

At Malbork Castle in Poland, the main entrance provides access to the Middle Castle. It is secured with five iron-bound gates standing one behind the other. Behind the first massive arch lies a wooden bridge that spans over a moat. Today it is static, but in the past it could be drawn up. Past this lies a portcullis, which could be lowered almost instantaneously if the castle was under threat. The ground is corbelled, and specially-laid stones marked the way for carts to they did not damage the walls. Malbork’s gates were designed to accommodate horses, but each was equipped with a smaller door that could be opened to admit those travelling by foot.

The term battlement is used to describe the top portion of castle walls. Most are crenellated, which means they consist of openings called embrasures alternating with sections of wall called merlons. Crenellations are designed to provide archers with cover and at the same time allow them a wide range of motion to fire at the enemy. In medieval England, castle builders were sometimes required to ask special permission before adding these defensive features to their fortifications.

Though crenellations predate the Middle Ages, they became much more elaborate and complex during this period. Originally, merlons tended to be rectangular in shape. Over time many different forms appeared, often influenced by regional architecture and culturally-specific design. Malaga’s distinctly-topped merlons serve as a poignant reminder of the Muslim peoples who raised this mighty castle. Due to their aesthetically-pleasing nature, crenellations continue to be constructed as decorative features even though they are militarily obsolete.