At first the means of protection were of the simplest kind. A wide earthen wall thrown up around a group of huts was regarded as a valuable defense. Stronger walls were made by using trunks of trees and rough stone work for the foundation and filling in the spaces with earth. Stakes were driven down and bound together to form a stout palisade, or fence. After a time wooden forts were reared of heavy logs and beams. Stone finally took the place of wood; and it was of stone that most of the castles of the days of knighthood were built. These were far removed from the simple fortifications of earlier times. They had massive stone walls and towers, moats, or wide, deep ditches filled with water, inner courts and outer courts, chapels, cellars, dungeons, together with chambers and staircases cut out of the thickness of the walls, drawbridges, and underground passages—all of which seem somewhat romantic in stories, but which were exceedingly necessary and matter-of-fact means of protection when they were built.

For the site of a castle a noble sometimes chose an island in a lake, like the famous Castle of Chillon in Lake Geneva, or a low, swampy place that an enemy would find difficult to reach; but he generally preferred the bank of a river or some high, rocky location. One of the most famous castles was the Château Gaillard, or the "Saucy Castle," which was built in Normandy by Richard the Lion-hearted in the days when kings of England still held possessions in France. It stood on a narrow promontory three hundred feet above the river Seine with a deep valley on either hand. The north end of the promontory was so steep and rocky that there was little danger of an attack on that side. The south end, however, sloped, and up this gently rising ground an enemy might easily advance. It was wise, then, to make the fortifications exceedingly strong at the south. A glance at the plan shows how this was done. C represents an outwork with five strong towers whose walls were eleven feet thick. These were connected by "curtains," that is, heavy stone walls from eight to twelve feet thick and thirty feet or more in height. All around this massive outwork was a ditch, E, some thirty feet wide and more than forty feet deep. The gate was at D; but before any one could reach it, he must find some way of crossing the moat. Friends might cross by means of a wooden drawbridge; but at the first glimpse of an enemy, chains and weights were set in motion, and the bridge was pulled up flat against the wall. The gate was protected by a portcullis, that is, a sort of screen made of heavy beams, each one pointed with iron. When no enemy was at hand, this hung quietly above the entrance, but at the first sign of danger, there was a great rattling of chains, and in a moment the portcullis had dropped in its grooves.

Between this outwork, or "outer court," C, and the "middle court," B, there must have been some sort of passage way, and probably walls to protect it. The middle court had also a moat. It had towers and curtains, and within it was a chapel, F, and a well, G. Out of this middle court an area was taken about as large as the outer court to form the "inner court," A. The wall which separated the two courts was so strong that it does not seem as if it could ever have been overthrown, for it was thirty feet high and eight feet thick. This was only the beginning of its strength, however, for on the side next the middle court rounding buttresses had been added. On top of the wall there were probably battlements, that is, a low, narrow wall running along the outer edge of the main wall and cut down at points a few feet apart. The defenders of the castle could shoot their arrows through the open spaces and then step behind the parapet for shelter. The wall protected the inner court, but the wall itself was protected, for the solid cliff on which it stood was cut down perpendicularly, or "scarped" for twenty feet, so that, even if an enemy had succeeded in getting possession of the middle court, he would still have the moat, H, to cross; and on the other side of the moat there would tower up above him twenty feet of perpendicular cliff and thirty feet of solid wall.

The entrance to the inner court was at I. It was approached by a causeway cut out of the solid rock; but it did not afford a very agreeable entrance to an unwelcome visitor, for there was an outer portcullis and an inner portcullis; and even if he succeeded in passing these and also the gateway studded with iron, he would find himself at the foot of a steep stairway cut in the rock, and the greeting that he would receive from the inmates of the castle would not be to his liking.

The inner court, then, stood some twenty feet above the middle court. It was protected by a wall thirty feet high with a perpendicular base twenty feet high and by a moat. Within this court was a deep well, for in case of a siege the defenders of this court might be cut off from the well at G. In this inner court was the strongest fortification of all, the castle proper, the great tower known as the keep, K. Its walls were eleven feet thick. The circular space within was twenty-six feet in diameter. In the basement was one window, but no door. The first floor had two windows, but they were small, for safety was thought of before air and sunshine. Here, however, was a door, small and well protected. It was many feet from the ground and was probably reached by a ladder or movable stairway. The keep was of a singular shape. Evidently King Richard thought that there was little danger of an attack being made from the west, for on that side was a sheer descent of cliff; but the spur of the keep that projected into the first court he made in the shape of a right angle and built it of solid masonry. The keep was the final place of refuge, and even after every other part of the fortifications had fallen into the hands of an enemy, this could generally withstand any attack that could be made by the engines of those times. Nevertheless, in order to make this keep even stronger, the lower part of the wall "battered," that is, it sloped outward at the base, while above the base rose what are known as machicolations. These were long, heavy brackets supporting a sort of gallery with a parapet. In the floor of the gallery between the machicolations were openings through which arrows could be shot downward or heavy stones could be dropped, or boiling water or oil or melted lead could be poured straight down upon the heads of the besiegers. It is thought that from the top of this keep another and smaller tower rose, and from that yet another, both probably built of wood.

Joining the keep on the north was a building, K, which is thought to have been the lodging of the castellan, or governor of the castle. From this building stairs descended to what was called the postern gate, L. This was a little narrow door with heavy bars. It was from this gate that spies or messengers were sent out in time of siege. To reach it from within, steps were cut in the rock for about thirty feet. To reach it from without must have been almost impossible, for it opened upon the perpendicular face of the scarp. To let out a messenger or admit a friend, a ladder or a movable bridge was let down. Every castle had its postern, so that if the inmates were besieged, they might have some possible way of communication, dangerous as it was, with the outer world.