When you descend into the O-bunker by the long outside stairway, you will notice a crowned monogram of King Frederik IX, above the year 1953. That is the year the installation was built. It wasn’t, however, commissioned until 1954. The designation “O-Bunker” is an abbreviation of “Operations Bunker”. In times of crises, but also during peacetime, this bunker was the centre of all military operations. In times of war, it acted as the fire-control room for the large-calibre artillery, but also for the close-range defence of the fortress. In peace-time, the main purpose was surveillance of shipping. The bunker was occupied day and night. In times of crisis the crew would consist of 25 men, the peace-time crew was about half that number.
Before one enters through the heavily armoured door, which had to remain closed at all times, you will see a loop-hole, around which there are several wooden frames which were meant to catch enemy bullets. The loop-hole was part of the close-range defence of the bunker and the gunner behind it had to guard the entrance. Next to the entrance there is a small toilet and beside that is the telephone exchange. The operator of the switchboard connected incoming and outgoing ‘phone calls. The walls of the exchange are covered with sound-proof material. Pistols were locked-up in a small cupboard behind the operator. The next room is the “BoN-room” for the close-range defence. There are several maps and boards on the walls, one of which contained the weather-forecasts of the Danish airfields and a map on which the positions of enemy troops around the fortress could be plotted. The switches for the closed circuit TV-cameras are also operated from this room. The wireless-room is situated behind the BoN-room. From here, the operators could maintain contact with other mobile units of the air-force and the navy. The room behind that is the cryptography room. When one uses the word “cryptic” in everyday life, one means that something does not make sense. And that’s exactly what is meant here. Messages – in military jargon ‘signals’ – could not be sent in clear language, because then the enemy would have been able to read them. So the signals had to be encoded: made illegible for others. Only someone who possessed the code-key could decode the message. This system was surrounded by great secrecy. The operator, who was entrusted with coded messages, was isolated in his own little room. We can see the sergeant behind a telex-machine. Radio signals could easily be transmitted several times using punch-tapes. They were stamped “Secret” or “Top Secret” according to their classification. A tube through the wall formed the connection by which signals could be passed from the code-room to the duty-officer in the neighbouring Operations room. This room is occupied by a large table - the so-called plotting table or ‘plot’ – which bears a large-scale map of the Baltic Sea and the Danish coastal waters. Next to the plot we see a sailor holding a crayon marker, ready to mark information concerning movements of enemy units on the map. The commanding officer is visible at a desk behind him. On his left we see several boards, one of which bears the positions of NATO-ships: Olfert Fischer, Grønsund, Falster etc. another contains data of ships of Eastern Block nations, i.e. Russian, Polish and East German. In the NATO-jargon these are designated as “Skunk”, “Friendly” or “Neutral”. The position, speed and course of all Eastern European ships were registered on the plot. They were regularly updated according to notices from coastal radar stations, friendly warships and aircraft. The crew of the operations room consisted of a maximum of 5 men, the commanding officer, two wireless operators and two plotters who operated the plot and the radar.