Old Main is the red-roofed, castle-like landmark on the Texas State University hill. It opened in 1903 as the school's first building, and its position above town still gives San Marcos one of its clearest silhouettes.
You do not need a long visit. Walk the campus, look back toward downtown, and treat it as part of the same town story as the square and the river: a college place shaped by steep hills, limestone, and spring water.