The oldest parts of the district of Old San Juan remain partly enclosed by massive walls. Several defensive structures and notable forts, such as the emblematic Fort San Felipe del Morro, Fort San Cristóbal, and El Palacio de Santa Catalina, also known as La Fortaleza, acted as the primary defenses of the settlement which was subjected to numerous attacks. La Fortaleza continues to serve also as the executive mansion for the governor of Puerto Rico. Many of the historic fortifications are part of San Juan National Historic Site.