A good location for a castle was on a natural rise, near a cliff, on the bend of a river, or where older fortifications such as Roman walls could be usefully reused. Castles needed their own water and food supplies and usually a permanent defensive force, additional factors to be considered when choosing a location.

Historically, the "best" location for a castle was determined by the dual needs of visibility and defensibility, making a high hill or rocky promontory the primary choice. Building on an elevation, such as a "motte," forced attackers to struggle uphill while exhausted and exposed to projectiles. Strategic geography was also vital; a site overlooking a narrow river crossing, a mountain pass, or a bustling harbor allowed the lord to control trade and collect tolls, which funded the castle’s upkeep. Access to a freshwater well within the walls was the single most critical factor for surviving a long siege, as a castle without water would fall within days regardless of its wall thickness. Builders also sought proximity to stone quarries to reduce the massive costs of transporting materials. In 2026, historians note that many iconic castles, like Edinburgh or Stirling, were built on "volcanic plugs"—extinct volcano cores that provided nearly vertical cliffs on three sides, making a successful frontal assault almost impossible for medieval armies.