The Romans left a mark on northwestern Spain that can still be seen today in bridges, ancient villages, gold mines, a lighthouse, and, most spectacularly, the Roman Walls of Lugo. I visited the latter when hiking parts of the Camino Primitivo in May 2019. This is one of two World Heritage Sites I visited on that trip to Spain that were famed for their walls (the other, Avila), and I have to say I preferred these walls. I circumnavigated Lugo twice, once following the perimeter at ground level, and once on the ramparts. Both trips gave a good perspective of the walls' height and width, which really must be seen to be appreciated. In the evening, quite a lot of locals come out for exercise jogging or walking around the walls. There's really not a lot more to say about this site -- the walls are remarkably intact and they so far have stood the test of time.
The history of Lugo, however, gave me a greater appreciation for how this corner of the Roman empire fits into world history. Lugo was likely originally named after the Celtic god Lugus, since Gallaecia, now known as Galicia, was home to the Gaels who settled Ireland (and, in fact, you can still hear bagpipes played in Galicia). The town was renamed Lucus Augusti when the Romans came to Galicia to mine gold for use in jewelry, art, and coins. The town became the most important for the Romans in Galicia, and in the 3rd century the Romans constructed the walls around Lugo not just to protect against outsiders, but also against local tribesmen unhappy with the Romans. They ultimately failed, as the Romans were defeated in the fifth century. Over time Lugo became a stop on the original pilgrimage route to Santiago de Compostela, and it still gets visits today from both pilgrims and enthusiasts of Roman architecture. The walls are certainly worth a visit if you find yourself in northwest Spain.
Logistics: The Roman Walls of Lugo are quite easy to access and traverse on foot.