According to a blueprint provided in the comic book (G.I. Joe #22, Marvel series), it's rebuilt to six stories in depth, depicted as having been designed to be more defensible and impregnable.
ɐkˈoːɹdɪŋ tʊ ɐ blˈuːpɹɪnt pɹəvˈaɪdᵻd ɪnðə kˈɑːmɪk bˈʊk dʒˈiː.ˈaɪ. dʒˈoʊ hˈæʃ twˈɛnti tˈuː, mˈɑːɹvəl sˈɪɹiz, ɪts ɹᵻbˈɪlt tə sˈɪks stˈoːɹiz ɪn dˈɛpθ, dᵻpˈɪktᵻd æz hˌævɪŋ bˌɪn dɪzˈaɪnd təbi mˈoːɹ dᵻfˈɛnsᵻbəl ænd ɪmpɹˈɛɡnəbəl.