The pivot of the coast defences was to be incumbent on the artillery acting in close cooperation with the land and sea-forces. The artillery was to engage at long range all the allied boats and to keep them under a constant crossfire, while they were endeavouring to proceed towards the coast. For that purpose, about sixty batteries, thirty five of which of the heaviest type were placed either along or at the rear of the sandhills.
In fact, the guns of very high caliber were distributed behind the belt of dunes; their tactical duty was to keep off the coast all boats which would try to carry out long range bombardments. As those batteries could not from their emplacements use direct fire, being unable to see the targets, observation-posts with good field of view on the sea—for fire registering—were found necessary. Herein lies the reason of the construction of the numerous concrete ones, some of them still remaining hidden along the downs.
As far as the smaller guns are concerned, these were dug in along the dunes and were performing direct fire to prevent against raids, attempts of landing or bottling the harbours.
Against eventual landings, the defences also comprised a series of mine barrages, screens of nettings for submarines, hindering the access of the shore. Redoubts manned by infantry troops were staking out an almost uninterrupted line of trenches which were running along the beach, right up to the Dutch frontiers.
Machine-guns and field guns were utilized for flanking the strand and for anti-aircraft action.
The German defence of the Belgian coast was completed by high sea forces.
The organization of the maritime defences was only seriously taken in hand in 1915. The “Boat-detachement” composed of trawlers, mine sweepers, tow boats and barges, was formed up in February that year. Small types of torpedo-boats and submarines were sent dismounted, and conveyed by railway up to the dock-yards created at Bruges, Ostend, and later on at Hoboken, where those units were mounted. They formed with a certain number of other units which came directly by sea, a flotilla of submarines and one of torpedo-boats. Motor trawlers were on the watch far out in the sea.
In 1916, the naval forces anchored in the Flanders base were considerably increased in number, but it was particularly in the year 1917 that the Germans multiplied their submarines, thus turning the harbours of Ostend and Zeebrugge into real haunts. Shelters were built to protect them against air raids and the numerous bombardments carried out by the British Fleet.
At Zeebrugge, the shelters were established along the mole and in the tidal basins south of the locks and in the Bruges canal.
At Ostend, the flotillas were berthed in the wharfs and the darses. The submarines were sheltered alongside the marine docks.