[2] Dating back to defense expansions following attacks by the English in 1598 and the Dutch in 1625, it was first built in 1634 as the Garita del Diablo ("devil’s sentry box") and an artillery platform.

After close to a hundred years of relative peace in the area, this section of the fortification (about a third) was demolished in 1897 to help ease the flow of traffic in and out of the walled city.

As a result of O'Reily's reforms, the Revellín de San Carlos (Saint Charles Ravelin) and the counterguard of La Trinidad (The Trinity) were built.

The defenses proved their effectiveness since in 1797 they helped to repel the invasion of seven thousand English soldiers commanded by the British general Ralph Abercromby.

1539 – Construction of the first fortified defenses at Castillo San Felipe del Morro and La Fortaleza, with batteries aimed at the harbor entrance.

1595 – The English attack San Juan, led by Sir Francis Drake, 25 ships penetrated the line of fire from El Morro.

[8] 1766–1783 - Main period of construction of San Cristóbal as we see it today under the directions of Royal Engineers Tomás O’Daly and Juan Francisco Mestre.

1898 – On 10 May 1898, the first shot which marked Puerto Rico's entry into the Spanish–American War was ordered by Captain Ángel Rivero Méndez is against the USS Yale from Castillo San Cristóbal's cannon batteries.

1942 – Still an active military base when the United States entered World War II, concrete pillboxes and an underground bunker control center are added to the ancient defenses of the Castillo San Cristóbal.

1961 – The US Army moves out of the forts of Old San Juan, and they become the jurisdiction of the United States National Park Service, to be preserved solely as museums.