During the third century B.C., the art of war made rapid progress so the defense tactics were improved. Romans and Greeks already used gateways in the defense of a fortress; curtain walls and towers were also common. The Romans governed for so long and occupied a vast empire. They left rectangular buildings measuring about 60ft by 70 ft called milecastle. Examples of their fortifications can be seen in Italy, Palestine, Gaul and Britain.
About 500 A.D. the Byzantine fortifications of this period are among the greatest works of military engineering of all time.
The medieval castle originated in the ninth century in France, western Germany and northern Italy because the nobles began building fortifications in response to increasing insecurity in the region.
Castles can be divided into two big categories: Medieval and Post-Medieval