El Viejo San Juan (Old San Juan) is the historic, colorful, and walkable old city of San Juan, Puerto Rico, known for its 16th-century Spanish colonial architecture, cobblestone streets, and impressive fortifications like El Morro and San Cristóbal forts. As the oldest city in the U.S. and its territories, it's a vibrant cultural hub with historic landmarks, plazas, shops, and restaurants, all located on a small island connected to the mainland by bridges.