The Porta Pia can be visited
Porta Pia

In Rome's ancient Aurelian Walls, the Porta Pia, a gate, was named after its initiator, Pope Pius IV. This structure, built between 1561 and 1565, replaced the nearby Porta Nomentana.
History+
The construction of the Porta Pia was commissioned by Pope Pius IV between 1561 and 1565, with designs by Michelangelo. It served as a replacement for the Porta Nomentana, located a few hundred meters to the south, which was closed at the same time. On September 20, 1870, the Bersaglieri breached the wall not far from the gate, marched into Rome, and completed the unification of Italy.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://en.wikipedia.org/wiki/File:RomaPortaPiaFacciataInterna.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 41.9092, 12.5012