DEFENSE IN DEPTH - San Cristobal was considered a masterpiece of 18th century military engineering, as can be seen in the "0utworks" which protected the land approach to San Juan. A maze of small forts, moats, tunnels, gunpowder magazines, trenches and mining galleries extends to the east, layer after layer, in a form that would make any attempt to take the fort by storm most difficult. Some of San Cristobal's outworks were dynamited in 1897 to allow the old colonial city to expand.