 Complete the following phrase:  At the outset, the defence system was based on the installations of the pre-war permanent fortifications, repaired and re-equipped in the years 19461954. 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 19531964, especially those built at the time of 