The series of passages and rooms at Guernsey's Castle Cornet were used as recently as the 1990s but have never been seen by the general public.
Helen Glencross, from Guernsey Museum, said the site was last used as a listening post by the Royal Navy.
German forces also occupied the site directly beneath the castle's citadel during World War Two.
Ms Glencross said the Ministry of Defence had taken over the rooms and tunnels following the war but it was not clear what they were used for.
The Royal Navy took on the lease of the citadel from 1974 to 1994.
Ms Glencross said: "We know the Royal Navy was using this position as a listening post and also a control and command post to marshal ships that were travelling to UK ports."
Guernsey Museum said it had removed about nine tonnes of rubbish from the site, which originates from the 13th Century.
The rooms will be open from Friday.
Rooms used by the UK's Ministry of Defence during the Cold War are being opened to the public.