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The walls were strengthened to be 18–40 feet thick. El Morro's improvements included 3 cisterns under the main plaza containing 216,000 gallons of water collected from times of rain. San Juan became a Defense of the First Order, and one of the most powerful plazas in the Americas by 1790. In 1765, Alejandro O'Reilly, Inspector General of Cuba, and Colonel Tomás O'Daly, San Juan Chief of Engineers, agreed on a plan to strengthen San Juan's defenses, which was approved by Charles III of Spain. By 1650, the town was enclosed on the east, south and west, while natural battlements protected the city along the Atlantic. In 1634, construction of the city walls surrounding San Juan began. A pair of batteries behind the hornwork overlooked the sea and harbor.
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The gate and drawbridge were protected by a ravelin, and just inside the gate was a guardhouse. Two half- bastions, one on the Atlantic side called "Tejeda", and another on the harbor side called "Austria", were connected by a curtain wall fronted by a moat, and spanned by a drawbridge in the center. The new fortifications consisted of a hornwork, crossing the headland, to protect the landward side of the existing tower and water battery. Pedro de Salazar took over construction in 1591. Captain General Diego Menéndez de Valdés, who was the governor of Puerto Rico, took over construction after Tejeda and Antonelli left to start construction of Santo Domingo's fortifications.
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San Juan construction began in March 1589 with skilled artisans, 12 stonecutters, 18 masons, 2 smiths, a cooper, metal founder, and an overseer assigned to the task, with the help 150 slaves. The plan, which was based on the then firmly established Spanish military fortification principles of the time, included fortifying nine other sites in the Spanish Main and Spanish West Indies: Santo Domingo, Santa Marta, Cartagena, Nombre de Dios, Portobelo, the Chagres River, Panama City, Havana, and St. It was not till 1587, however, that Field Marshal Juan de Tejeda and the Italian engineer-architect Juan Bautista Antonelli drew the fort's final design. It is estimated that this section comprises about 10% of the whole structure. In order to have a viable defense while the rest of the fort was being completed, a small proto-fortress was erected during the first year of construction. The original fortress was built under the direction of conquistador Diego Ramos de Orozco and its main purpose was to defend the port of San Juan by controlling the entry to its harbor. The construction of the citadel and its surrounding walls began in 1539 on orders of King Charles V of Spain. Facing the structure, on the opposite side of the bay, a smaller fortification known as El Cañuelo complemented the castillo's defense of the entrance to the bay. Over two million visitors a year explore the castillo, making it one of Puerto Rico's leading tourist attractions. In 1983, the citadel was declared a World Heritage Site by the United Nations in conjunction with the San Juan National Historic Site. The fortification, also referred to as el Morro or 'the promontory,' was designed to guard the entrance to the San Juan Bay, and defend the Spanish colonial port city of San Juan from seaborne enemies. Lying on the northwesternmost point of the islet of Old San Juan, Castillo San Felipe del Morro is named in honor of King Philip II of Spain.