Throughout the Middle Ages, Europe’s rulers built strongholds along the frontiers of their realm to guard against the constant threat of invasion. The earliest castles were earth-and-timber forts located along trade routes or likely avenues of attack. As kingdoms grew in power and wealth, these forts were replaced by massive permanent structures with thick stone walls.

The first requirement for a castle was that it be difficult to reach, and the easiest way to do that was to build in a naturally inaccessible location. Dunnottar Castle in Scotland, for example, sits on a bluff near the sea, leaving attackers few lanes of approach. Other castles were built on islands, or steep hilltops. Where there was no natural defense, builders created one by digging a moat around the castle. Some moats were filled with water diverted from a nearby river, but most were not, as even a dry moat presented a formidable obstacle. The only access over the moat was a bridge, which could be drawn into the castle. Large gatehouses guarded entry to the bridge.

The castles themselves boasted ingenious defenses. The battlements, which give castle walls their familiar notched silhouette, afforded shelter as archers returned fire through “arrow slits” cut into them.  Many battlements were also built with overhangs called machicolations. These had holes in the floors so the defenders could drop stones or hot oil on any invaders who reached the castle walls. If any enemy did manage to get inside the castle, he would have to dodge stones dropped through “murder holes” cut into the ceilings.

Though castles were first and foremost defensive structures, they also served as residences for the lord and his family. Inside, elaborate vaults and buttresses created soaring interiors that were often described as elegant as they were strong. A castle had to house not only its master, but also a garrison of soldiers and the retinue of servants required to keep it running. To meet the huge demand of services and supplies, entire towns often sprang up around the castle.