Tight Mesh or Narrow Gaps: This is one of the most common and effective features. The fence panels are made with a very small mesh aperture (e.g., "358 mesh" which refers to a 3" x 0.5" x 8 gauge mesh). This tiny spacing doesn't allow for a person to get a hand or foothold, making it virtually impossible to scale.
Lack of Horizontal Rails: Traditional fences often have horizontal rails that act as rungs on a ladder. Anticlimb fences, such as palisade fencing, are often designed with only vertical bars, providing no steps for climbing.
Smooth Surfaces: The materials used, whether it's tightly welded mesh or vertical steel bars, often have a smooth, slick surface that offers no friction or grip points.
Angled or Curved Tops