Programme began in 1938 following an order by Hitler to strengthen fortifications on the western German border. Limes refers to the former borders of the Roman Empire; the cover story for the programme was that it was an archaeological study. Its Type 10 bunkers were more strongly constructed than the earlier border fortifications. These had thick ceilings and walls. A total of 3,471 were built along the entire length of the Siegfried Line. They featured a central room or shelter for 10–12 men with a stepped embrasure facing backwards and a combat section higher. This elevated section had embrasures at the front and sides for machine guns. More embrasures were provided for riflemen, and the entire structure was constructed so as to be safe against poison gas. Heating was from a safety oven, the chimney of which was covered with a thick grating. Space was tight, with about per soldier, who was given a sleeping-place and a stool; the commanding officer had a chair. Surviving examples still retain signs warning "Walls have ears" and "Lights out when embrasures are open!" Aachen-Saar The Aachen-Saar programme bunkers were similar to those of the Limes programme: Type 107 double MG casemates with concrete walls up to thick. One difference was that there were no embrasures at the front, only at the sides of the bunkers. Embrasures were only built at the front in special cases and were then protected with heavy metal doors. This construction phase included the towns of Aachen and Saarbrücken, which were initially west of the Limes Programme defence line. Western Air Defence Zone The Western Air Defence Zone (Luftverteidigungszone West or LVZ West) continued parallel to the two other lines toward the east, and consisted mainly of concrete flak foundations. Scattered MG 42 and MG 34 emplacements added additional defence against both air and land targets. Flak turrets were designed to force enemy planes to fly higher, thus decreasing the accuracy of their bombing. These towers were protected at close range by bunkers from the Limes and Aachen-Saar programmes. Geldern Emplacement The Geldern Emplacement lengthened the Siegfried Line northwards as far as Kleve on the Rhine, and was built after the start of the Second World War. The Siegfried Line originally ended in the north near Brüggen in the Viersen district. The primary constructions were unarmed dugouts, but their extremely strong concrete design afforded excellent protection to the occupants.