What factor would most likely influence a medieval king in selecting a location for a castle?

The choice of natural barriers as a significant factor for a medieval king in selecting a location for a castle is rooted in the strategic advantages these features provided. Natural barriers, such as mountains, rivers, or forests, served as obstacles to potential invaders, making it more difficult for enemy forces to approach the castle. These geographical features were essential for defense, enabling a king to fortify his position and protect his territory against attacks.

Additionally, the presence of natural barriers could enhance the visibility of approaching threats, giving defenders early warning and allowing them to prepare for a siege. A castle situated near a river might also benefit from control over water routes, adding to its strategic importance.

While trade routes, population density, and climate conditions could influence the selection of a castle's location, they are not as directly tied to the primary purpose of a castle, which was defense. Trade routes might facilitate commerce, while population density could reflect resources or labor availability, and climate conditions could affect comfort and sustainability. However, in terms of immediate tactical considerations, natural barriers remain the most crucial factor for a medieval king when deciding where to build a castle.