Complex sentence: It was finished by 1944 and was connected to the "Vorbunker" by a stairway set at right angles (not a spiral staircase) which could be closed off from each other by a bulkhead and steel door.
Split into simpler sentences: It was finished by 1944 and was connected to the "Vorbunker" by a stairway set at right angles (not a spiral staircase). The two bunkers could be closed off from each other by a bulkhead and steel door.