Porta Ternana stands as a testament to late 15th-century military engineering. Constructed between 1471 and 1492, it served as a vital fortified bastion, strategically positioned where the eastern city walls met the ascent to the Rocca di Albornoz. Its imposing presence is largely due to the distinctive diamond-pointed ashlar stone used for its door jambs and arch, a decorative yet functional element that adds to its austere majesty. The gateway's design incorporates defensive features such as arrow slits and gun ports, remnants of its past role in protecting the city from invaders approaching via the ancient Via Flaminia.

Originally known as Porta delle Arvolte, the gate's structure included flanking towers, some of which were semicircular rooms that housed garrison soldiers. These spaces provided shelter and a vantage point for guards overseeing the entrance. A significant modification occurred in 1832 when the gate was raised by approximately two meters, altering the visual proportion of the ashlar work relative to the battlements. Despite this alteration, Porta Ternana, alongside the Rocca, remains one of Narni's most impressive ancient military structures, offering visitors a tangible connection to the town's historical defenses.

Travelers often remark on the sheer scale and preservation of the gate, noting how it immediately transports them back in time. The detailed stonework and the visible defensive elements are frequently highlighted as points of interest, making it a popular spot for photography and historical reflection.