In ancient time when the fortress curtain wall was made of earth excavated from a ditch in front of the wall, towers, if they were used, were constructed of wood. Previously we have seen different styles of towers, open backed, accessed only over small drawbridges in the curtain wall from the walkway or Allure. They were also constructed attached to buildings inside the fortification, or built standing alone, or constructed as solid towers supporting the curtain wall. Often they were used to defend the corners of the castle wall as well as to lend support the defenders with the use and housing of heavy artillery as well as an ammunition store. As this skill in castle building developed the problems with certain aspects of tower construction were brought to the fore and solved by the master-builders of the day. Square towers created "dead angles" which left the towers open to the enemy's mines, . The Roman engineers had already made this determination and we see round and "D"-shaped towers often incorporated into their fortress designs. These kinds of towers which sometimes projected out from the curtain wall were known as "flanking towers." These flanking towers, allowed the fighting men to cover the "flanks" or sides of the curtain wall easily. Use of towers along a curtain wall, including the style, shape, and spacing; all were determined to a great extent by the geography and terrain on which the castle / fortress was constructed.
The material of the early towers was of wood which, of course, is easily subject to fire, and so it was not long before stone was used in their construction over the raw animal hides that covered wooden towers. This took a longer period of time to build, but was a more lasting medium with which to build. Tower design often revealed the amount of money available to build such fortifications, and the design also depended greatly upon the master-builder or designer who laid out the plans for the fortifications.
A special note about a feature adopted by European castle designers which originated in the Middle East. This was the "plinth" (a thickening and outward sloping base to the tower) which was a stabilizing factor for the tower as well as making it more difficult to engage in mining under the towers. This idea was, in later castles, also adapted to the curtain walls as well as the towers.
The Keeps (a massive chief tower in ancient or medieval castles / fortifications) often were used as residences with floors inside the keep and stairways inside the walls for access. Towers were an important part of castle design from Roman times through to the 13th and early 14th centuries.