The history of Germany's North Sea coastal defence is a military history oddity. The geography was a natural ally of the German navy as the Russian winter was to Russian armies.
This satellite imagery shows most of the German North Sea coast; the missing part on top is similar. We can easily see the chain of islands which leaves only the Eiderstädt peninsula on top truly exposed. Everywhere else "brown water" is an understatement. Muddy bottoms and very shallow waters are everywhere and restrict capital ship movements (see also this training nautical map). Ports and cities are thus well-protected from hostile surface actions.
|typical look of the shore|
|The North Sea withdraws far during Ebb tides,|
leaving a muddy landscape
It was relatively easy to secure the few navigable channels with coastal artillery, but 20th century wartime defences rested heavily on mines and motor torpedo boats. Minimally armed fishing boats (Kriegsfischkutter) served as picket boats (Vorpostenboot). Hostiles also had to fear false buoys, as they could lead ships into too shallow waters. The smaller navigable channels change over time, and years-old nautical maps are unreliable.
There's but one extant coastal fort; it guarded the entrance to the Elbe river estuary and thus to the primary German harbour city, Hamburg. Fort Kugelbake (satellite image). It was equipped with ten 283 mm guns before the First World War, half of these were moved to Flandres in 1914 already, and the fort only served as heavy anti-air battery with very little coastal defence value during the Second World War.
English North Sea coast cities endured some shelling during the First Word War; the German ones never did. The Royal navy had a plan for an amphibious invasion of Germany prior to the First World War already, (Baltic project), but it involved the risky navigation through the Danish waters and a landing on the German Baltic Sea shore - the North Sea shore was too troublesome.
The Sweden, Finland and Estonia - have a similar oddity in that the Baltic Sea is has little salt and thus often freezes over for months during wintertime. Norway has its unique fjord coastline, Sweden its archipelago in front of its capital. Croatia has a similar coastline as Germany, sans the influence of the tides. Much of the Dutch coastline resembles the German one.
Of all these countries, only Norway suffered from a (quite daring) amphibious invasion (Estonia experienced an invasion of its islands only).
I always thought this geographic influence on naval military history was quite interesting, and now I finally shared it.
P.S: Coastal artillery had historically a rather low lethality because its high lethality was usually avoided (save for few exceptions). Duels between cruisers and coastal forts were typically indecisive in both World Wars. Most coastal artillery lacked the battery size for effective long-range fires. (As a rule of thumb it takes a six salvo for a good observation of the centre of impact fountains and thus for accurate observed and corrected fire at long ranges. This is why the move from four to six or more primary artillery guns with the All-big-gun battleship in the Dreadnought revolution was such a big deal.) Most large calibre coastal guns were often very slow-loading compared to shipboard guns because the means for ammunition transport were more primitive. Coastal mortars were almost useless with their low rate of fire.