The fort is **Castillo San Felipe del Morro**, construction began 1539. Key to Spanish defense against naval attacks, protecting the entrance to San Juan Bay for over 400 years. The prominent cylindrical structure is the **lighthouse** (El Faro del Morro), built in 1908. Replaced previous lighthouses dating back to 1846; the current design is a U.S. Army Corps of Engineers build, contrasting with the original Spanish stonework. The imposing walls, built of local **coralline limestone**—*piedra muca*—provided exceptional protection, capable of withstanding cannon fire. The layers visible in the photo show centuries of weather and fortification improvements. The small, ornate structure embedded in the wall is an old **sentry box** entrance. Sentry boxes (*garitas*) were vital lookout posts, iconic in Puerto Rican architecture and often featured in local legends and folklore.