▁Arab ic ▁houses ▁sometimes ▁have ▁a ▁" B ab ▁ir r ": ▁a ▁secret ▁door ▁used ▁as ▁an ▁emer gency ▁exit ▁built ▁into ▁the ▁walls ▁and ▁hidden ▁with ▁a ▁window ▁s ill ▁or ▁a ▁book case . ▁The ▁name ▁comes ▁from ▁one ▁of ▁the ▁six ▁gates ▁cut ▁through ▁an ▁ancient ▁wall ▁in ▁A den ▁( in ▁modern - day ▁Y emen ), ▁which ▁was ▁opened ▁only ▁in ▁the ▁event ▁of ▁a ▁state ▁security ▁emer gency . ▁In ▁modern - day ▁Spain , ▁the ▁Arab ▁for tr ess ▁of ▁Ben qu eren cia ▁has ▁a ▁Bab ▁al - S ir r ▁known ▁as ▁the ▁" Do or ▁of ▁Tre ason ." ▁ ▁Other ▁secret ▁pass ages ▁have ▁sometimes ▁been ▁constructed ▁after ▁the ▁initial ▁building , ▁particularly ▁secret ▁t unn els . ▁These ▁t unn els ▁have ▁often ▁been ▁created ▁as ▁escape ▁routes ▁from ▁pr isons ▁or ▁prisoner - of - war ▁cam ps , ▁where ▁they ▁are ▁known ▁as ▁escape ▁t unn els . ▁These ▁secret ▁t unn els ▁typically ▁require ▁a ▁hidden ▁opening ▁or ▁door , ▁and ▁may ▁also ▁involve ▁other ▁de cept ive ▁construction ▁techniques , ▁such ▁as ▁the ▁construction ▁of ▁a ▁false ▁wall . ▁Other ▁t unn els ▁have ▁been ▁made ▁for ▁different ▁reasons , ▁such ▁as ▁sm ugg ling ▁t unn els ▁used ▁for ▁sm ugg ling ▁fire ar ms , ▁illegal ▁dru gs , ▁or ▁other ▁cont rab and . ▁ ▁History ▁There ▁have ▁been ▁many ▁instances ▁throughout ▁history ▁of ▁secret ▁pass ages ▁and ▁rooms ▁having ▁been ▁used :