This national historic landmark was settled in 1830 by former residents of Revilla (now Nuevo Guerrero, Tamaulipas) under the leadership of Jesús Treviño. The site was in the southwest corner of the original Hacienda de Dolores, a grant made in 1750 by Col. José de Escandón to José Vázquez Borrego, and was named for the patron saint of Guerrero, Saint Ignatius Loyola. In 1830 Treviño built a sandstone home, known as Fort Treviño, 100 by 140 feet. José Villareal placed a sundial at the home in 1851; the timepiece has become a tourist attraction. San Ygnacio became a center of trade by the mid-1800s, and the town was the scene of several border skirmishes throughout the years. 

Like many frontier buildings of its era, the fort is fortified by strong walls that protect a courtyard within. Its walls have no windows, only small gunports through which those inside the fort could fire at attackers outside. The fort sits on a small bluff above the Rio Grande, and the lush vegetation and plentiful birdsong creates an almost tropical feeling.