The Dombunker, located in the northwest of Calais, was constructed by German forces in July 1940 to shelter railway-mounted artillery. Its nickname—'cathedral bunker'—refers to the shape of its ogival roof, reminiscent of Gothic cathedral architecture. Measuring 80 metres in length, 12 metres in width, and 10 metres in height, with 1.5-metre-thick concrete walls and armoured doors 20 cm thick, it was further concealed from aerial reconnaissance with camouflage netting.

This structure was part of Stp 89 Fulda (formerly Stp 151 Feige), a fortified position featuring two rail-mounted K5 280 mm Krupp guns, among other armaments. The site also included bunkers for personnel and ammunition, anti-aircraft defences, and rotary firing platforms (Vögele) that allowed the railway guns to pivot 360 degrees for targeting flexibility. Nearby positions, such as Fort Nieulay to the west and Calais citadel to the east, formed part of the larger coastal artillery network. Today, only the Dombunker, an ammunition shelter, and a small personnel bunker remain visible.

The artillery sheltered by the Dombunker belonged to the Eisenbahn-Artillerie-Abteilung 725 (Railway Artillery Battalion 725), a unit created under the Sofortprogramm to strengthen German coastal defences against the United Kingdom. By July 1942, this unit operated several batteries across the French and Belgian coast. At Calais, Batterie E.2/725 was equipped with two K5 guns capable of long-range bombardment, targeting the English coast including Dover.

Commanded successively by officers such as Hauptmann Konrad Günther and Major Anton Pohlmann, the unit coordinated with other batteries positioned between Dunkirk and Abbeville. As of 6 June 1944, multiple railway artillery batteries remained in the Pas-de-Calais region to defend against potential Allied landings. In September 1944, as Canadian forces advanced, Batterie E.710, based at the Dombunker, retreated to Sluskil in the Netherlands. Hindered by Allied air attacks and unable to withdraw further, the battery destroyed its own guns and was disbanded.

The Dombunker is one of the few remaining structures linked to the German railway artillery programme, often locally referred to as the shelter of the “Grosse Bertha.”