La Fortaleza and San Juan National Historic Site in Puerto Rico

Massive walls and forts guard the harbour mouth at Old San Juan, built up layer by layer from the 16th to the 20th century as Spain — and later the United States — defended this key to the Caribbean. El Morro and its companion works show European military engineering rethought for an American harbour. UNESCO inscribed La Fortaleza and the San Juan fortifications in 1983.