San Cristobal in Old San Juan, Puerto Rico is an 18th-century masterpiece of military engineering and innovation. It took over 150 years to build by the Spanish and this fortress was used by the United States Army in WW11 for bunkers and coastal observation posts. It's truly a fascinating and awe-inspiring destination, especially for history buffs. San Cristobal's Walkway instigates one's imagination, yet is delivered in a simple, minimalist approach.