could only enter and leave using special permits, were not permitted to other villages within the zone, and were subjected to nighttime curfews. It was not fenced off but access roads were blocked by checkpoints. On the far side of the Sperrzone was the signal fence (Signalzaun), a continuous expanded metal fence long and high. The fence was lined with low-voltage electrified strands of barbed wire. When the wire was touched or cut, an alarm was activated to alert nearby guards. Protective strip On the other side of the signal fence lay the heavily guarded "protective strip" (Schutzstreifen), wide, which adjoined the border itself. It was monitored by guards stationed in concrete, steel and wooden watchtowers constructed at regular intervals along the entire length of the border. Nearly 700 such watchtowers had been built by 1989; the larger ones were equipped with a powerful 1,000-watt rotating searchlight (Suchscheinwerfer) and firing ports to enable the guards to open fire without having to go outside. Their entrances were always positioned facing towards the East German side, so that observers in the West could not see guards going in or out. Around 1,000 two-man observation bunkers also stood along the length of the border. Guard dogs were used to provide an additional deterrent to escapees. Dog runs (Kettenlaufanlagen), consisting of a suspended wire up to long to which a large dog was chained, were installed on high-risk sectors of the border. The dogs were occasionally turned loose in temporary pens adjoining gates or damaged sections of the fence. The guards used an all-weather patrol road (Kolonnenweg, literally "column way") to patrol the border and travel rapidly to the scene of an attempted crossing. It consisted of two parallel lines of perforated concrete blocks which ran beside the border for around . Next to the Kolonnenweg was one of the control strips (Kontrollstreifen), a line of bare earth running parallel to the fences along almost the entire length of the border. There were two control strips, both located on the inward-facing sides of the fences. The secondary "K2" strip, wide, ran alongside the signal fence, while the primary "K6" strip, wide, ran along the inside of the fence or wall. In places where the border was prone to escape attempts, the control strip was illuminated at night by high-intensity floodlights (Beleuchtungsanlage), which were also used at points where rivers and streams crossed the border.