At the outset, the defence system was based on the installations of the pre-war permanent fortifications, repaired and re-equipped in the years 1946–1954. After 1950, due to the increased tension between the Eastern and Western Blocs, a more sophisticated system of pillboxes and shelters was built. While the pre-war blockhouses and pillboxes were designed as monoliths of reinforced concrete, the new cold-war bunkers followed the Soviet paradigm and were more like reinforced field fortifications, built from stone and prefabricated concrete elements. Many of the installations from the period 1953–1964, especially those built at the time of Berlin and Cuban crises, were designed for conditions of nuclear war, and many older installations were given enhanced protection against weapons of mass destruction.