The Rocca Pia is the squat four-towered fortress that guards the southern entrance to the old town, built in 1461 by Pope Pius II to enforce papal control over what was then a fractious independent commune. The bare rusticated walls and round corner towers are characteristic of mid-Quattrocento military architecture.
The interior served as a prison from the 1860s until the 1960s, and the cells with their inmate graffiti are now part of a small museum. The roof terrace gives the best overall view of Tivoli's old centre.