The Castro name derives from the Latin *castrum* — a fortified place — and the family traced its origins to Castrogeriz, the ancient hilltop town on the Camino de Santiago in the province of Burgos. They belonged, alongside the Lara, the Haro, and the Guzmán, to the five great houses linked by blood to the first kings of Castile. The Castro-Lara rivalry — a bitter contest for influence over the Castilian crown during the reign of Alfonso VIII — defined twelfth-century court politics before the family's centre of gravity shifted permanently westward.

By the thirteenth century, the Galician branch of the house had eclipsed the Castilian line. Through a series of strategic marriages — most critically to the royal house of León — the Castros established themselves as Lords of Lemos and Sarria, controlling the great fortified towns of interior Galicia from Monforte de Lemos to Castro Caldelas. The historian Murguía called them a "semi-royal dynasty"; Crespo Pozo observed that no other Galician house could claim so many blood ties to the medieval kings of Spain. It was not an exaggeration. The Castros descended from Alfonso IX of León, married into the line of Sancho IV of Castile, and produced children who would sit on — or shake — the thrones of both Castile and Portugal.

Pedro Fernández de Castro "el de la Guerra" fathered four children who would shape the history of two kingdoms. By his wife Isabel Ponce de León, he had Fernando — who would become the last great Castro lord of Galicia — and Juana, who married Pedro I of Castile. By his mistress, a Portuguese noblewoman named Aldonça Lourenço de Valladares, he had Álvaro — who would become Constable of Portugal — and Inês.

Inês de Castro arrived at the Portuguese court around 1340 as a lady-in-waiting to Constance of Castile, the bride of Prince Pedro, heir to the Portuguese throne. Pedro fell in love with the Galician noblewoman. The affair endured beyond the death of Constance in 1345, producing four children, and drew the Castro family — through Inês's brothers Fernando and Álvaro — dangerously deep into Portuguese court politics. King Afonso IV, fearing the Castro clan would drag Portugal into the Castilian succession crisis, ordered Inês killed. On 7 January 1355, three courtiers murdered her at the monastery of Santa Clara in Coimbra.

When Pedro became king in 1357, he hunted down two of the killers and had their hearts torn from their bodies. He then announced that he had secretly married Inês before her death, legitimizing their children. According to the legend that has never left the Portuguese imagination, he ordered her body exhumed, dressed in royal robes, seated on the throne, crowned, and the entire court compelled to kiss the dead queen's hand. The twin tombs at the Monastery of Alcobaça are real. Pedro and Inês lie face to face across the nave, placed so that, according to legend, they will see each other first when they rise on the Day of Judgment. The daughter of a Galician lord of Lemos and Sarria became the most famous queen in Portuguese history. The phrase *Agora é tarde; Inês é morta* — "It's too late; Inês is dead" — remains a common Portuguese expression to this day.

In 1370, Enrique II sent his enforcer west. Pedro Ruiz Sarmiento — a Castilian nobleman with no Galician roots, carrying the title Adelantado Mayor de Castilla — arrived in Galicia at the head of the feared *Compañías Blancas*, the French mercenary companies that had helped Enrique seize the throne. His mission was simple: destroy Fernando de Castro and break the last Petrist resistance in the kingdom.

Fernando gathered an army of Galician loyalists and Portuguese allies and rode to meet him. The two forces clashed at Porto de Bois, near Palas de Rei in the province of Lugo, in early 1371. The site held a grim family resonance — Fernando's grandfather had been killed in battle at the same place in 1304. History repeated itself. The Castros were routed. Fernando fled south across the Miño into Portugal, wounded and broken.

The Treaty of Santarém in 1371 forced the Portuguese to expel the Petrist exiles. Fernando retreated to the castle of Ourém, then to English-held Bayonne in Gascony, where he died in 1377 — the last lord of the greatest Galician noble house, buried in exile in a foreign city. His tomb bore the inscription: *Aquí iace Don Fernando Ruiz de Castro, toda la lealtad de España.* The man who replaced him — Pedro Ruiz Sarmiento — received the Adelantamiento Mayor de Galicia from Enrique II and, in 1375, the lordship of Ribadavia. The lands the Castros had controlled or influenced for two centuries passed to newcomers. Porto de Bois was not just a battle. It was the end of a world.

The arms of the Galician branch of the Casa de Castro bear six blue roundels (*roeles de azur*) on a silver field (*argent*), arranged 2-2-2. The Portuguese branch used thirteen roundels on gold. The roundel — called a *torteau* in French blazon and a *roel* in Castilian — is among the oldest charges in Iberian heraldry, and the Castro arms are considered one of the most ancient armorial devices on the peninsula. Over time, as the house merged with the Osorios to form the Castro-Osorio line that held the County of Lemos in the sixteenth and seventeenth centuries, the arms were quartered with the Osorio wolves passant — but the six blue roundels remained the identifying mark of the original house.

Murguía called the Castros a "semi-royal dynasty." Hermida Balado went further: "the only lineage that could have forged a dynasty of kings in Galicia." The arms survive on churches and pazos across the interior of Galicia — from Monforte de Lemos to Castro Caldelas to the ruins of lordly houses in the Ribeira Sacra. When the senior Castro line failed for lack of heirs, the County of Lemos passed through the Osorios and eventually to the House of Alba, where the title rests today. But the six blue roundels on silver — the oldest heraldic mark of Galician lordship — endure on stone lintels and church walls throughout the kingdom the Castros once ruled.