The height of walls varied widely by castle, but were often 2.5–6 m (8.2–19.7 ft) thick. They were usually topped with crenellation or parapets that offered protection to defenders.

The thickness of castle walls in the Middle Ages varied significantly depending on the century and the specific strategic importance of the wall, but they were generally between 7 and 20 feet thick. In the early Norman period, stone walls were often 5 to 8 feet thick. However, as siege technology advanced—particularly with the introduction of the counterweight trebuchet—walls became increasingly massive. For example, the curtain walls of Edward I’s "Iron Ring" castles in Wales often reached 10 to 12 feet in thickness. The most vulnerable parts of the castle, such as the base of the keep or the gatehouse, could be up to 20 feet thick at the foundation to prevent "sapping" or mining (where attackers would dig under the wall to make it collapse). These walls were rarely solid stone; they were usually "rubble-filled," consisting of two outer layers of finely dressed stone (ashlar) with a core of rough stones, flint, and a very strong lime mortar, which allowed the wall to absorb the impact of projectiles without shattering like a solid monolithic block would.