By 1650, the stone walls enclosed the city on the east, south and west sides. The new Castle, which was at this time little more than a redoubt, protected the entrance to the city from the east. El Morro protected the seaward approach to the north.

In the 17^th century, the new Castle acquired the name of San Cristóbal, after the hill upon which it was built. Rising almost 150 feet above sea level on the north eastern side of Old San Juan, about a mile from San Felipe del Morro, it is the largest fortification in the Americas and comprises two major parts, the main Castle and the outer defenses.

When construction of the Castle commenced in 1634, it was in the form of a small triangular redoubt. Day after day, 400 men, laborers, prisoners, soldiers and some slaves, toiled on the Castle and the city walls. These walls were completed in 1785, by which time San Cristóbal had been transformed into a system of fortifications covering almost 11 hectares. Each of the Castle’s structures were interconnected so as to form multiple lines of defense. If the enemy captured one line of defense, the remainder could continue fighting; this feature is known as defense in depth.

The illustration shows the Castle as it was in 1790. We see the main Castle and its hornwork, giving continuity to the walls of the city. In front of the hornwork are the ravelin of San Carlos and the Trinidad counterguard, both surrounded by dry moats. Beyond the moat is the parade ground, which leads to a fortification known as El Abanico (the fan). Looking towards the sea from El Abanico one can see Santa Teresa, the battery defending the north coast, and La Princesa, whose cannons can defend against attacks from both land and sea. Underground galleries had been excavated beneath the esplanade, and most of these still exist today. Other important structures of the Castle including the east wall and the ravelin and bastion of Santiago, were demolished to extend the city eastwards in 1897.

The highest part of San Cristóbal is called the "Caballero", a large platform for cannons constructed above the hornwork. From this level, the city and the whole bay could be observed. At the foot of the hornwork (the oldest wall in the Castle) we found the second level or main battery, with embrasures dominating the east side of San Juan. Descending from the second level via the main artillery ramp, we arrive at the interior courtyard or main square. This was the place where the soldiers paraded in the mornings to receive their orders of the day and conduct their military exercises. In this square we see the chapel of Santa Barbara (the patron saint of artillerymen) and also the infantry barracks, the officers’ quarters, kitchens, latrines, powder magazines, the guardhouse, prisons and wells. As in the case of El Morro, cisterns are located below ground level in the main square to catch and store rainwater, the only source of drinking water at the time. There are five such cisterns in San Cristóbal and three in El Morro.

A comprehensive system of underground tunnels was constructed to allow the movement of troops, artillery and powder. There was also a system of "mined galleries", explosives placed in underground tunnels, which could be detonated beneath an advancing enemy to deny them access to the interior of the Castle or the city. The completion of San Cristóbal and the city walls, (considered to be a masterpiece of military engineering in the 18^th century), made San Juan the most fortified city in the Caribbean.