Castrelo de Miño sits on the left bank of the River Miño, the great artery of Galicia, in the heart of the Ribeiro wine comarca of Ourense province. The municipality encompasses 39.74 square kilometres of gentle hills, mountainous reliefs, and extensive vineyards that cascade down sheltered slopes toward the river. Its name derives from the Latin "castrum" — a small fortress — recalling the ancient Celtic fortification, Castrum Minei, that once commanded the river crossing.

The landscape is defined by its relationship with the Miño. The Castrelo Reservoir, created when the river was dammed in 1968, now stretches over 10 kilometres, fundamentally transforming the terrain from fertile agricultural valley to a vast inland lake flanked by vineyards and forest. The municipality lies at the intersection of Atlantic and Mediterranean climates, with mountains providing a protective microclimate ideal for viticulture — warm summers averaging 14.5°C annually, with 1,915 hours of sunshine.

Roman conquest in the late 1st century BC brought Castrelo de Miño into the province of Gallaecia, transforming the landscape through mining, road-building, and the introduction of systematic agriculture. Gold mining operations were established at Los Cotos (between Prado and Astariz), Monte Rosario (Macendo), and the Lavadero de Prado de Miño. Four Roman roads crossed the territory, connecting Castrelo to Ourense, Arnoia, Celanova, and beyond — roads that would serve as medieval pathways for centuries.

The most extraordinary discovery came in 2016-2017, when archaeologists from the University of Vigo excavated the Castro de Santa Lucía in Astariz. Among circular Castro dwellings and Roman-era structures, they uncovered a Galician-Roman rock-cut wine press (lagar rupestre), dated using a coin from 235 AD. This proved that vine cultivation and winemaking in Castrelo de Miño dates back to at least the 1st-3rd centuries AD — making it one of the oldest documented wine-producing sites in all of Galicia. The Romans also established thermal baths at El Diestro, exploiting the natural hot springs that bubble up along the Miño.

Two fortified positions guarded the river crossing at Castrelo — one at the monastery site, another across the Miño. This strategic crossing became the stage for some of the most dramatic episodes in Galician medieval history. In 1110-1111, following King Alfonso VI's death, civil war erupted between supporters of his daughter Queen Urraca and those backing her young son Alfonso. Count Pedro Froilaz de Traba seized the fortress of Castrelo de Miño and installed the six-year-old prince under his protection. When Archbishop Diego Gelmírez of Santiago de Compostela arrived to negotiate, he was briefly arrested by Arias Pérez. Released shortly after, Gelmírez personally crowned the boy as King Alfonso VII of Galicia on September 17, 1111 in Santiago de Compostela Cathedral. A famous legend tells of an eagle that warned the Archbishop of the betrayal.

A decade later, in 1121, Queen Urraca had the Archbishop arrested again at Castrelo de Miño while he was returning from Portugal — episodes documented in the Historia Compostelana. The medieval bridge, reputedly built by Saint Peter González ("San Telmo") and said to have had eight arches, was destroyed by flooding in the mid-16th century when a massive walnut tree lodged in one of its arches. The 15th century brought the Irmandiño revolts (1467-1469), when approximately 80,000 peasants, fishermen, and artisans rose against the feudal nobility across Galicia, transforming the social order.

The Church of Santa María de Castrelo de Miño is the municipality's most important monument — built on the very site of the ancient Castrum Minei, where the monastery once stood. Its origins trace to the 10th century, and the surviving Romanesque apse from the 12th century is exceptional: semicircular, divided by three attached columns, with windows of semicircular arches. The corbels and metopes feature extraordinary reliefs depicting Christ, Saint Peter and Saint Paul, alongside monks, dogs, wolves, an eagle, and a donkey. The tower, unusual for the region, shows curious similarities with the Romanesque of Catalonia and Lombardy.

Inside the church, 16th-century Renaissance murals (c. 1560) depict the Last Judgement, Christ before Pilate, the Flagellation, and the Road to Calvary — the vices personified as a procession heading toward the jaws of Hell. The church was rebuilt in Baroque style in 1763. Other notable monuments include the Baroque Church of Santa María de Astariz with its 1748 altarpieces by sculptor Antonio Salvador Carmona, the Romanesque Church of Prado de Miño, the Church of Santo Estevo de Ponte Castrelo, and the manor houses Pazo de Troncoso and Casa da Capela.

Castrelo de Miño's thermal springs have been known since Roman times, when baths at El Diestro served travellers along the Miño valley. By 1772, the Baths of Santa María were formally constructed over the remains of a Castro-Roman settlement, featuring two springs: the Burga Alta (60°C) and the Burga de Abaixo (47°C). The sulphurous-chloride-sodium waters were contained in small granite bathing pools, just 178 centimetres long and 50 centimetres deep. In 1888, these waters represented Ourense province at the 1888 Barcelona Universal Exposition.

When the reservoir was filled in 1968-1969, the thermal baths of Santa María vanished beneath the waters — but remarkably, the springs remain active. During dry seasons when the reservoir level drops, the original stone pools emerge from the lake and can still be used, the hot water bubbling up as it has for millennia. Local authorities have installed wooden walkways to facilitate access during these periods. The Termas de O Diestro near the power station also remain accessible, their curative waters a living link to the Roman past. Ourense province, known as the "Capital of Thermalism" in Spain, counts these among its most evocative thermal sites.