German WW1 fortification

German High Sea fleet

With the destruction of the Russian Baltic Fleet in the Toshima Strait during the Russian-Japanese War in 1904, Germany had become the largest naval power in the Baltic Sea region.
The most important naval port in the Baltic was Kiel, which was protected against any attacks of the English Home Fleet. In addition, the Germans had a significant number of naval ships ports on the North Sea coast.

The 15 years leading up to World War I, Germany had commenced an almost hysterical naval arms race with England. The ships could not get big enough, guns could not be big enough and not enough of them could be mounted on the ships. This ended with ships af the Dreadnaught class.
It was initiated by Admiral Tirpitz and basically Germany had not the resources for this.

After the Battle of Jutland may 31th 1916, which both Germany and England claimed to have won. The High Seas Fleet (Hochseeflotte) retreated to the ports of the North Sea, Wilhelmshaven and Cuxhaven  and Kiel in the Baltic.

U-boats KIel 1914

Meanwhile, the U-boat weapon, which had great success in the Atlantic, had been heavily reinforced .
At this time both the Germany army and the population suffered under the Brittish blokade of the North Sea harbours.   
Germany was forced to take some initiative outside the trenches, and blocking the English supply chain over the Atlantic was a possibility. 
This led the German government and supreme command to declare unrestricted U-boat war february 1st 1917. This meant that all ships, neutral or from belligerent nations, were sunk on suspicion of cargo that Germany considered contrabande. 

Germany was aware that this decision was unpleasant  for the two neutral old shipping nations the Netherlands and Denmark. They both bordered on Germany, If they entered the war on the English side because of the submarine war, they would constitute some uncomfortable flank. However, the German supreme command did not think this would happen.
They were more nervous about American entry into the war, but at the same time they were so desperate to stop supplies to England that they were prepared to run this risk.

Hindenburg and Ludendorff

Denmark, which was only 100 km. North of the Kiel naval base, and Hindenburgs and Ludendorffs concern arose. At one point there had been unconfirmed rumors that England had promised France a landing of 100,000 at Esbjerg, just north of the then border, if an French-German war should occur.
From here they were to advance towards Kiel and further south with Berlin, possibly even supported by the Danish army.

This led to Germany to start construction of a stronghold across Jutland, in the at the time German territory North Schleswig in autum 1916. This was territory Germany occcupied from 1864 to 1920. 
The position starts east at the Sliv Lake between Haderslev and Aabenraa and runs via Toftlund and Skärbäk to the coast of the island of Römö in the West.
Sicherungsstellung Nord.

Sicherungsstellung Nord

Over 900 molded plants, trenches, barbed wire and a lot of light, medium and heavy batteries.
The stronghold was never manned, but the army units were designated just as there were designated units  to take action at the Dutch border if necessary.