A perimeter is a structure that circles the perimeter of an area to prevent access. These fences are frequently made out of single vertical metal bars connected at the top and bottom with a horizontal bar. They often have spikes or Barbed wire on the top to prevent climbing. Residential perimeter fences are normally made of wood about six inches thick, or a solid wall. Some fences may be string-like pieces of metal stretching across horizontally and vertically, creating a mesh of metal.