As an additional bomb-proofing method, the dome was surrounded by a bomb-proof "skirt" or "Zerschellerplatte" of steel-reinforced concrete, wide and thick. This was supported by a series of buttresses, which were not tied into the dome itself, above the entrances to the "Gustav" and "Gretchen" tunnels. Another concrete structure was tied into the skirt to the north-west of the dome, which was perhaps intended for use as an observation and control tower. A separate underground building was constructed on the western side of the quarry to serve as a hospital and as offices for the engineers. A Decauville narrow-gauge railway was installed on the quarry floor to transport supplies from the main line to the construction site.