Defensive towers come in several different forms. In castle building, the first of these structures appeared as part of the wooden palisades that fortified motte and bailey castles.
Some towers are integrated into the walls. Others are structurally independent and flanked by stretches of curtain. Certain designs project out to make it easier for archers to cover adjacent stretches of wall, others have an open back so the tower can be readily supplied via crane from below, or targeted from behind if it falls to the enemy. Some towers even have a thicker, sloped base to make undermining more difficult.
Examples of tower positioning relative to curtain wall. Sourced from The Medieval Fortress by Kaufmann and Kaufmann.
Originally, castle towers were predominately square. Unfortunately, this configuration leaves the structure vulnerable in areas that archers can’t cover.
Diagram depicting releationship between tower shape and possible angles of defensive fire. Sourced from The Medieval Fortress by Kaufmann and Kaufmann.
In the 12th century, medieval engineers begin to design circular and semicircular towers to solve this problem. Towers could also augmented with arrow slits and/or machicolations to make them more readily defensible.
To see the Towers at these castles, check out our blog Castle Features #5 - Towers