Puerto Rico's Colonial Style architectural heritage noticeable in the old city of San Juan, is heavily influenced by the island’s four hundred years under Spanish rule, from its discovery in 1493 until the US Occupation in 1898. It is evident in the massive fortresses and military installations built by the Spanish Royal Corps of Engineers to defend the city against invaders. Some of these structures, like the wall built around the city only partially remain, but other structures like the ones listed below, remain almost intact.

• San Felipe del Morro Fortress named in honor of King Phillip II, is a massive six level 16th-century citadel built between 1540 and 1589. Rising 140 feet above the sea, its 18-foot-thick wall proved a formidable defense.  It fell only once in 1598 to a land assault by George Clifford, 3rd Earl of Cumberland.  Most of the walls in the fort today were added between 1760 and the 1780's.  The fort is a maze of tunnels, dungeons, barracks, outposts and rampa studded with small, circular sentry boxes or garitas.  Its seventy four acre area represents the largest fortification in the Caribbean.  El Morro as it is commonly named, was designated a National Historic Site in February 1949 and is administered by the US National Park Service.  In 1992, for the 500th anniversary celebration of the discovery of Puerto Rico by Christopher Columbus in his second voyage, the fortress was restored to its historical architectural grace and beauty.

• Fort San Cristóbal is El Morro's partner in the city's defense on the opposite end of the Atlantic shoreline of Old San Juan.  Its construction began in 1634 and was completed in 1771.  It is supported by a massive system of outworks which provided defense in depth and is one of the largest defenses ever built in the Americas.  It rises 150 feet above sea level and covers twenty seven acres of land.  As if its size and height weren't sufficient to intimidate enemies, its intricate modular design was sure to foil them.  Its strategic design is outstanding  with five independent units, each connected by  a series of moats and tunnels, each fully self-sufficient should the others fall.  It's a World Heritage and National Historic Site, administered by the US National Park Service.  

• La Fortaleza or the Palace of Santa Catalina was authorized by King Charles V and built between 1533 and 1540 as a fortress to defend against Carib Indian attacks. The building was the first of a series of military facilities constructed in the Bay of San Juan, but later proved inadequate to guard the entrance to the harbor and became the official Governor's Residence.  Today, it is the oldest governor's mansion still used as such in the Western Hemisphere.  It has been occupied twice by invaders; by George Clifford, 3rd Earl of Cumberland in 1598 and by Dutch corsair and later Admiral Bowdoin Henrick in 1625 when the building was damaged by fire.  A major reconstruction was undertaken in 1640 and in 1846 the building was remodeled and given a palatial aspect, uniting harmoniously its 16th century military architecture with the refinements of the 19th century.  It has been the home of 170 governors of Puerto Rico and is the official residence of the current governor.  Although the original palatial building surrounded by gardens was very primitive, La Fortaleza has undergone numerous changes over its 400 years history.

• Fort San Gerónimo is a small fort in the line of defense of San Juan that was built to replace a small fortification that was destroyed during the 1598 attack led by George Clifford, 3rd Earl of Cumberland.  It is located in the Puerta de Tierra neighborhood at the entrance of the Condado Lagoon, just behind the Caribe Hilton Hotel.  Its location allowed for the fort to protect the entrance to the lagoon and block access to the San Juan islet from the east.  San Gerónimo Fort played an important role in defending San Juan during the attacks by the English and the Dutch. Even though it suffered a lot of damage in every attack, it was always restored and improved.

• The Escambrón Battery is a small fortification located at the eastern tip of Escambrón Beach in the Tercer Milenio Park near the San Gerónimo Fort.  It is not as old as the other San Juan fortifications as it was built at the end of the 18th century, but it was an integral part of the line of defense of San Juan and served as a shooting practice place for the Spanish Army.  The Escambrón Battery is not part of the San Juan National Historic Sites.

• Fortín San Juan de la Cruz aka El Cañuelo was a wooden fort originally built on its current site in the late 1500s which burned to the ground in 1625 during a Dutch attack. However, the Spaniards replaced it with a stone fort between 1630 and 1660 which still stands today. El Cañuelo was the smallest fort in the harbor defense system. Cannon fire from the fort and from Castillo San Felipe del Morro created a crossfire to protect the entrance to San Juan bay.

​The pictures below include the fortresses listed above and structures in and around Old San Juan that are noteworthy designs, mostly built in the 19​th Century during the Spanish colonial era and some after the US occupation in 1898.  Some notable architects of these structures are Fernando Montilla Jimenez (1870-1929) and Miguel Ferrer Otero (1887-1966), Adrian C. Finlayson the US Department of the Interior State Architect until 1921, Raúl Reichard (1908-1996), Osvaldo Toro and Miguel Ferrer of the firm Toro & Ferrer and Francisco Roldán Martinó (1890-1988).

Also pictured below are designs by noted Puerto Rican architects Juan Bertoli Calderoni​, ​Elias Concepción, Julio Vizcarrondo, Agustin Camilo Gonzalez, ​Julio G. Conesa Revoredo (1873-1930), Francisco Porrata Doria, Blas Silva Bouchert, Alfredo Weichers Pieretti, Pedro Adolfo de Castro Besosa, Manuel V. Domenech, Rafael Carmoega and Pedro Mendez.  ​​ Probably the most prolific of these was Porrata Doria, the ebook Los Templos de Francisco Porrata-Doria by José Mari Mutt available free on the Apple Books application, details the many churches he designed throughout the island.

Clicking on the pictures below will enlarge the image or direct you to a related website with more information on the structure or the architect.