The end of Paseo de la Princesa marks the transition to Paseo del Morro. This National Recreation Trail begins below La Fortaleza. The mid-16th century fortress still serves as the executive mansion for the governor of Puerto Rico. The walking path follows the bay at the base of the city walls. These three miles of masonry are called La Muralla. Some sections measure 20 feet thick and 40 feet tall. The defenses were constructed from 1634 until 1782. This sentry post is next to the San Juan Gate. La Puerta de San Juan was built in 1634, making it the oldest of the original five gates (puertas) into Old San Juan. It was also the formal entrance. Countless Spanish dignitaries arrived here starting in 1540.