San Cristóbal and the eighteenth-century fortification boom

Castillo San Cristóbal was begun in 1634, originally as two minor works called the Caballero de San Miguel and the Garita del Diablo, raised directly in response to the 1598 and 1625 attacks described in the previous lesson. It grew into the largest fortification the Spanish built anywhere in the New World.

The fort kept growing for some hundred and fifty years more, especially under King Charles III, gaining outer ravelins and additional batteries as military engineering advanced. It reached its overall greatest extent in 1783, a scale of construction that reflected how seriously Spain took the ongoing threat against San Juan.

The finished fortress rises some 150 feet from the Atlantic shore as a three-level structure built specifically to guard the land access to Old San Juan, complementing El Morro's own defense of the harbor entrance from the open sea.

A census taken in this same era of fortification, conducted in 1765 by Lieutenant General Alejandro O'Reilly, recorded a total island population of 44,883, of whom 5,037 people, about 11.2 percent, were enslaved: a comparatively low proportion next to other Spanish Caribbean colonies of the period, a figure explored further in a later lesson on the island's sugar economy.

The fortifications were tested one last time in 1797, when Britain made its final major attempt to take the island. General Ralph Abercromby led an invasion force of some 7,000 soldiers and 64 ships against San Juan, but the defenders of Captain General Ramón de Castro repelled the British, who withdrew for good. San Cristóbal was designated a UNESCO World Heritage Site in 1983, along with El Morro and La Fortaleza.

Sources: Castillo San Cristóbal (San Juan); Historia de Puerto Rico. Freeze 2026-08-22.