Fortification Plan of El Morro, citadel of San Juan, Puerto Rico

Plan of El Morro in 1742, being the citadel of San Juan, Spanish Puerto Rico (Instituto de Historia y Cultura Militar, Madrid). As it can be seen on the map, El Morro was a small peninsula in a strategic location protecting the harbour of San Juan.

Puerto Rico was almost for four centuries one of the main strongholds of Spanish power in the Americas (Wikimedia commons).