The original screenplay was written in Galician, but the two female leads (Marian Álvarez, Manuela Vellés) didn't speak the language, so the script had to be adapted to Spanish. There are still a few Galician words and expressions, and both leads met with a dialect coach before filming.
Miguel Peña is mentioned as a participant in "Andújar's siege". This combat happened in 1936-1937 in the Santuario de Nuestra Señora de la Cabeza (Sanctuary of the Virgin Mary of the Head), located in Andújar, Jaen (Andalusia, south of Spain) where Republican militias fought against Civil Guards and civilians for 9 months to conquer a strategical point in the Spanish Civil War.
Manuela's necklace is a trisquel, an ancient Celt symbol for good luck. It's the same symbol used in the TV show Charmed: Jovens Bruxas (1998), where it's completed with a circle inside, receiving the name of triquetra.
The movie is inspired by real events that happened in Galicia at the early 1940s, when General Franco allowed the Third Reich to explode some of his mines in Spain to extract mineral for creating tanks and weapons for Hitler's armies during Second World War.