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.