-Concentric castles were built. They consisted of having many lines of defense, meaning that if an invading army destroyed one line of defense, the castle would still have more. Some castles had three or four outer walls and the keep in the center.
-Edward I castles. They were situated near the sea. Making provisions easy to get.
During the Pre-Medieval Times, there were not what we know as "castles." Fortifications were common all across Rome. Towers were made of timber and they were situated in key-positions such as on top of a hill or in another place to watch a specific area.
Walls were used a lot. Many cities had a wall surrounding them as a line of defense. Dating from Syria and Egypt; walls have been used as a means to protect a village against a sudden attack. There are many records of villages taken over in a single night by an army. Walls and towers helped to stop this from happening.
Forts were used mainly by Romans, who, in need of protection for their troops; built them.
William the Conqueror created a sudden turn in Europe's warfare. In 1066, when he conquered England and was crowned king, he built more than 40 castles scattered throughout England. His successors continued doing what he started, and in a matter of years, hundreds of castles were built all around Europe.
Earlier castles were made of timber. Even though timber was strong against spears and arrows, fire could render it useless. Defenders, to protect their timber castles, had water ready to extinguish any fire. In posterior years, moats were built around a castle to protect it against quick invasions. Soon afterwards, defenders realized that timber castles were not efficient. They had many blind spots from which pickaxes could open a hole in the wall.
A battering ram could destroy the main gate in a matter of minutes and fire arrows could burn a castle completely in no more than a day. For this reasons, timber castles stopped being built around the XII century; giving room to more elaborated strongholds.
During this period of time, stronger castles were built. Stone was widely used for making castles since, even though cold, it was very effective against a siege.
Castles with many sides were built. Orford castle is very famous for being well preserved and for being the only 5-sided castle in England.
Still, it is a very clear example of a castle with many sides used for not having blind spots and much better accuracy for archers. During this period of time, castles were much higher. They were normally built on a hill, near a river or on another defensible position. If there wasn't castles would normally have many outer walls and a moat surrounding them. Circular-shaped towers were built to diminish blind spots.
Allures were implemented to castles and castle gates were improved greatly.
It was noted that castles were not effective enough against well-elaborated sieges. Most older castles couldn't resist more than a month of being besieged since they had a very small amount of provisions in the castle. Some older castles didn't even have a well! For this, and other reasons, concentric castles were built.
First, they were used to have many lines of protection in which there could be farmland, fruit trees and wells. Having sometimes more than four lines of defense dramatically improved a castle's chance of resisting a siege. If one of the castle's wall was to fall, archers could easily fire at the invading soldiers from the top of the keep.
After a few years of building concentric castles, Edward I realized that it wasn't enough for a castle to have farmland in the interiors, he then, built castles very near the sea or lakes so that food could be brought from the exterior--even when under a siege.