Construction began in September 1943 with the building of railway lines to support the work, and excavation of the gun shafts began in October.
The initial layout comprised two parallel complexes approximately 1,000 metres (3,300 ft) apart, each with five drifts which were to hold a stacked cluster of five HDP gun tubes, for a total of 25 guns.
The smoothbore design of the HDP would enable a much higher rate of fire than was possible with conventional guns.
The entire battery would be able to fire up to 10 shots a minute, capable in theory of hitting London with 600 projectiles every hour.
Both facilities were to be served by an underground railway tunnel of standard gauge, connected to the Calais–Boulogne main line, and underground ammunition storage galleries which were tunneled at a depth of about 33 m (108 ft).
The western site was abandoned at an early stage after being disrupted by Allied bombing, and only the eastern complex was built.

The drifts were angled at 50 degrees, reaching a depth of 105 m (344 ft).
Owing to technical problems with the gun prototype, the scope of the project was reduced; drifts I and II were abandoned at an early date and only III, IV and V were taken forward.
They came to the surface at a concrete slab or Platte 30 m (98 ft) wide and 5.5 m (18 ft) thick, in which there were narrow openings to allow the projectiles to pass through.
The openings in the slab were protected by large steel plates, and the railway tunnel entrances were further protected by armoured steel doors.
Each drift was oriented on a bearing of 299°, to the nearest degree – a direct line on Westminster Bridge.
Although the elevation and direction of the guns could not be changed, it would have been possible to alter the range by varying the amount of propellant used in each shot.
This would have brought much of London within range.

The railway tunnel ran in a straight line for a distance of about 630 m (2,070 ft) .
Along its west side was an unloading platform which gave access to ten cross galleries (numbered 3–13 by the Germans), driven at right angles to the main tunnel at intervals of 24 metres (79 ft).
Each gallery was fitted with a 600 mm (1 ft 11 ⁵⁄₈ in) gauge railway track.
On the east side of the tunnel were chambers intended to be used as store rooms, offices and quarters for the garrison.
Trains would have entered the facility and unloaded shells and propellant for the guns.

Galleries 6–10, the central group, gave access to the guns, while galleries 3–5 and 11–13 were intended for use as access tunnels and perhaps also storage areas.
They were all connected by Gallery No.
2, which ran parallel to the main railway tunnel at a distance of 100 metres (330 ft).
Galleries 6–10 were additionally connected by a second passageway, designated Gallery No.
1, running parallel to the main tunnel at a distance of 24.5 metres (80 ft).
Further workings existed at depths of 62 m (203 ft), 47 m (154 ft) and 30 m (98 ft), each serving different purposes associated with the drifts and the guns.
The 62 m workings were constructed to facilitate the removal of spoil from the drifts, while those at 47 m were connected with the handling of exhaust gases from the guns and those at 30 m gave access to the breeches of the guns.
The lower levels of the workings were accessed via lift shafts, and mining cages were used during construction.

The construction work was carried out by over 5,000 workers, mostly German engineers drafted in from several companies including Mannesmann, Gute Hoffnungshütte, Krupp and the Vereinigte Stahlwerke, supplemented by 430 miners recruited from the Ruhr and Soviet prisoners of war who were used as slave labourers.
The intensive Allied bombing campaign caused delays, but construction work continued nonetheless at a high pace underground.
The original plans had envisaged having the first battery of five guns ready by March 1944 and the full complement of 25 guns by 1 October 1944, but these target dates were not met.
Construction began in September 1943 with the building of railway lines to support the work, and excavation of the gun shafts began in October. The initial layout comprised two parallel complexes approximately 1,000 metres (3,300 ft) apart, each with five drifts which were to hold a stacked cluster of five HDP gun tubes, for a total of 25 guns. The smoothbore design of the HDP would enable a much higher rate of fire than was possible with conventional guns. The entire battery would be able to fire up to 10 shots a minute, capable in theory of hitting London with 600 projectiles every hour. Both facilities were to be served by an underground railway tunnel of standard gauge, connected to the Calais–Boulogne main line, and underground ammunition storage galleries which were tunneled at a depth of about 33 m (108 ft). The western site was abandoned at an early stage after being disrupted by Allied bombing, and only the eastern complex was built.
The drifts were angled at 50 degrees, reaching a depth of 105 m (344 ft). Owing to technical problems with the gun prototype, the scope of the project was reduced; drifts I and II were abandoned at an early date and only III, IV and V were taken forward. They came to the surface at a concrete slab or Platte 30 m (98 ft) wide and 5.5 m (18 ft) thick, in which there were narrow openings to allow the projectiles to pass through. The openings in the slab were protected by large steel plates, and the railway tunnel entrances were further protected by armoured steel doors. Each drift was oriented on a bearing of 299°, to the nearest degree – a direct line on Westminster Bridge. Although the elevation and direction of the guns could not be changed, it would have been possible to alter the range by varying the amount of propellant used in each shot. This would have brought much of London within range.
The railway tunnel ran in a straight line for a distance of about 630 m (2,070 ft) . Along its west side was an unloading platform which gave access to ten cross galleries (numbered 3–13 by the Germans), driven at right angles to the main tunnel at intervals of 24 metres (79 ft). Each gallery was fitted with a 600 mm (1 ft 11 ⁵⁄₈ in) gauge railway track. On the east side of the tunnel were chambers intended to be used as store rooms, offices and quarters for the garrison. Trains would have entered the facility and unloaded shells and propellant for the guns.
Galleries 6–10, the central group, gave access to the guns, while galleries 3–5 and 11–13 were intended for use as access tunnels and perhaps also storage areas. They were all connected by Gallery No. 2, which ran parallel to the main railway tunnel at a distance of 100 metres (330 ft). Galleries 6–10 were additionally connected by a second passageway, designated Gallery No. 1, running parallel to the main tunnel at a distance of 24.5 metres (80 ft). Further workings existed at depths of 62 m (203 ft), 47 m (154 ft) and 30 m (98 ft), each serving different purposes associated with the drifts and the guns. The 62 m workings were constructed to facilitate the removal of spoil from the drifts, while those at 47 m were connected with the handling of exhaust gases from the guns and those at 30 m gave access to the breeches of the guns. The lower levels of the workings were accessed via lift shafts, and mining cages were used during construction.
The construction work was carried out by over 5,000 workers, mostly German engineers drafted in from several companies including Mannesmann, Gute Hoffnungshütte, Krupp and the Vereinigte Stahlwerke, supplemented by 430 miners recruited from the Ruhr and Soviet prisoners of war who were used as slave labourers. The intensive Allied bombing campaign caused delays, but construction work continued nonetheless at a high pace underground. The original plans had envisaged having the first battery of five guns ready by March 1944 and the full complement of 25 guns by 1 October 1944, but these target dates were not met.
Fortress of Mimoyecques