A short walk from the city sits imposing Castillo San Felipe de Barajas, covering almost the whole of San Lázaro Hill, 135 feet above sea level. The castillo bit is something of a misnomer, as the structure is not technically a castle but a fort; it happens to be one of the most impressive the Spanish ever built, resisting a number of land and sea attacks. Allegedly its tunnel system was engineered so that the slightest sound anywhere within it would reverberate a warning of approaching danger or attempted escape. Audio guides, available in English, Spanish, and other languages, recount the full story. The castle also happens to be one of the best spots in the city from which to watch the sunset.