<P> The "fourth - generation Wall", known officially as "Stützwandelement UL 12.11" (retaining wall element UL 12.11), was the final and most sophisticated version of the Wall . Begun in 1975 and completed about 1980, it was constructed from 45,000 separate sections of reinforced concrete, each 3.6 metres (12 ft) high and 1.2 metres (3.9 ft) wide, and cost DDM 16,155,000 or about US $3,638,000 . The concrete provisions added to this version of the Wall were done to prevent escapees from driving their cars through the barricades . At strategic points, the Wall was constructed to a somewhat weaker standard, so that East German and Soviet armored vehicles could easily break through in the event of war . </P> <P> The top of the wall was lined with a smooth pipe, intended to make it more difficult to scale . The Wall was reinforced by mesh fencing, signal fencing, anti-vehicle trenches, barbed wire, dogs on long lines, "beds of nails" (also known as "Stalin's Carpet") under balconies hanging over the "death strip", over 116 watchtowers, and 20 bunkers with hundreds of guards . This version of the Wall is the one most commonly seen in photographs, and surviving fragments of the Wall in Berlin and elsewhere around the world are generally pieces of the fourth - generation Wall . The layout came to resemble the inner German border in most technical aspects, except that the Berlin Wall had no landmines nor spring - guns . Maintenance was performed on the outside of the wall by personnel who accessed the area outside it either via ladders or via hidden doors within the wall . These doors could not be opened by a single person, needing two separate keys in two separate keyholes to unlock . </P> <P> As was the case with the inner German border, an unfortified strip of Eastern territory was left outside the wall . This outer strip was used by workers to paint over graffiti and perform other maintenance on the outside of the wall Unlike the inner German border, however, the outer strip was usually no more than four meters wide, and, in photos from the era, the exact location of the actual border in many places appears not even to have been marked . Also in contrast with the inner German border, little interest was shown by East German law enforcement in keeping outsiders off the outer strip; sidewalks of West Berlin streets even ran inside it . </P> <P> Despite the East German government's general policy of benign neglect, vandals were not unknown to have been pursued in the outer strip, and even arrested . In 1986, defector and political activist Wolfram Hasch and four other defectors were standing inside the outer strip defacing the wall when East German personnel emerged from one of the hidden doors to apprehend them . All but Hasch escaped back into the western sector . Hasch himself was arrested, dragged through the door into the death strip, and later convicted of illegally crossing the de jure border outside the wall . Noted graffiti artist Thierry Noir has reported having often been perused there by East German soldiers . While some graffiti artists were chased off the outer strip, others, such as Keith Haring, were seemingly tolerated . </P>

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