A castle's first line of defence was the high, thick, outer wall that completely surrounded it. The tops of the walls, the battlements, usually had a jagged design known as crenellation: a series of gaps that allowed the castle's defenders to fire arrows from the tops of the walls on to the enemy troops below while providing cover when they reloaded their bows. There were also narrow slits in the walls through which archers could fire their arrows while remaining protected from return fire.

Walls

The walls between towers were called curtain walls. They could measure between three to five metres (10–16 feet) in thickness. The inner and outer faces were constructed first. The stones were fitted carefully into place. The space in between was then filled in with rubble mixed in with mortar—a mixture of sand, lime and water. Some walls were constructed with arched passageways running inside the walls, to enable guards to get from one section another hidden from enemy view. The walls were then limewashed (painted white) to protect against the rain and to give a smart finish.

Arrow loops

The narrow openings in the walls—too narrow for anyone to climb through—were called arrow loops. They were splayed inside so the archer could shoot comfortably but it was virtually impossible for the enemy to shoot back at them through the narrow holes. Cross slits were designed for crossbows.

Crenellations

The design of the tops of the walls and towers, the battlements, was a key part of the castle’s defences. Running along the top of the wall was a wallwalk, sometimes called an allure. Soldiers could use this to reach any section of the wall under attack.

Both the wallwalk and the top of the towers were protected from enemy fire by a higher wall called a parapet. This had gaps in it called crenels or embrasures, sometimes fitted with hinged covers allowing archers and crossbowmen to shoot their arrows while being protected from return fire. The solid parts of the parapet, the merlons, had narrow slits built in them, called arrow loops, which performed a similar function. The gap-tooth design of a castle’s battlements was called crenellation.

Hoardings

In preparation for a siege, additional defensive structures called hoardings could be fixed to the battlements. They were wooden frames that projected from the wall in front of the crenellations and were covered by roofs. Wet animal hides were nailed to them to prevent them from being set alight. The hoardings protected the defenders from arrows, while the floor had openings through which defenders could drop missiles.

Machicolations

Wooden hoardings caught fire easily and could be easily smashed by missiles hurled by the enemy's siege machines. So from about the 1270s, hoardings were rebuilt in stone. The parapet was built with a small overhang to the wall below. In the floor were square holes through which guards could shoot arrows or drop hot sand or missiles on to attackers below. These stone overhangs were called machicolations.