In the popular imagination, the idyllic college campus is a leafy, parklike place, with tree-lined walkways winding among venerable buildings of stone or brick, where students zip to class on scooters or find quiet spots to study on vast lawns.

But, in fact, millions of students in the U.S. and around the world attend colleges or universities not in a sylvan setting but in a major city. In the past, such campuses were often planned in a traditional, picturesque style and built like a city-within-a-city—gated and fenced, with their own power plants, utilities, and security forces.