A visit to Fort Houmet on Guernsey shows a small Victorian gun fort that had been adapted by the Germans to hold more modern weapons.
Visiting Fort Houmet
The exterior of the fort is freely available but the interior and chapel is open on Saturdays in the summer, information in the link below.
My Experience
We visited in February 2012 so couldn’t go inside.
Fort Houmet Photo Gallery
History
After the Wehrmacht occupied the Channel Islands on 30 June 1940, they assessed the existing defences to determine if they would be of use. The Germans found the Islands’ fortifications antiquated and woefully inadequate for modern warfare.
Because the Germans expected to invade the United Kingdom in the autumn of 1940, they decided that expenditure on defences for the islands would be a waste. Initially the Germans built only feldmässige Anlage (field-type construction) positions. By 1941 the prospect of conquering Britain had decreased and the probability of an eastern war increased, requiring defences to be built to reduce the number and quality of troops required to defend the western ocean areas. Whilst the Luftwaffe and Kriegsmarine had their roles in protecting the islands from the Allies, the occupying forces put their main effort into land defences aimed at repelling a seaborne or airborne assault.
The resulting construction work in the Channel Islands was extensive; it required thousands of workers and massive supplies of cement and steel. Tiny Guernsey received special treatment. It had the largest artillery pieces in the Channel Islands, tanks, and 12,000 troops: one soldier for every two civilians on the island, compared to France which had a 1:80 ratio, or higher.
Defensive areas were built to protect facilities or an area.
The concept of lineal defences having been discredited in World War 1, the current idea was hedgehog defences with all round visibility and support from other positions giving interlocking fire.
There were twelve Stützpunkt (Strongpoint) (SP) areas in Guernsey, such as Stützpunkt Rotenstein at Fort Hommet which comprised: 4×10.5 cm casemates, 2x 60 cm searchlights, an MG bunker, a 4.7 cm anti tank casemate, a tobruk pit mounted with a French tank turret, a type R633 bunker containing a M19 automatic mortar, and two other bunkers for personnel and storage as well as barbed wire, minefields, flamethrowers and trenches. A type R633 bunker took 845m³ of concrete and 40 tons of steel to build.
References
The history above is copied from the Wiki page on the fortifications of Guernsey here.
The visit Guernsey page has full visitor information for Fort Houmet here.
There are better pictures here.