Barbed wire
The fences were strengthened in the late 1950s, particularly in areas with a high number of escapes. Parallel rows of barbed-wire fences were built, with concertina wire added between the fences in some places to further hinder escapees. The outer fence was often located very close to the actual border. A further obstacle was added by placing short wooden anchor posts about outwards from both fences. Barbed wire was strung between them to form a V-shape to hinder lateral movement by escapees. In some low-risk areas, only a single fence was installed.