The Age of Heroes was a period in Klingon history when there were wars being fought worldwide around Qo'noS. Many military medical facilities made by the Klingon Empire were made to recapture the feeling of the deep underground built structures of that time period which were heavily armored with low ceilings as well as thick metal-clad walls. This was noted by Admiral Leonard McCoy during his attempt to save Teilani from a dangerous poison in her system. (TOS novel: Preserver)