Sinister atmosphere - created through haunting music and clever use of green landscapes that adds the feel of isolation - is packed in a well structured story that goes from spontaneous setup, classical plot and unexpected ending. Considering that it also brings a fair amount of gore, it comes as kind of underrated "should be" zombie classic, with bland action and generic characters being its only major flaws.
If I exclude the Romero's classics this is my favorite zombie horror film. The classic Hammer's "Plague of the zombies" comes close with the trashy / cheesy "The dead next door". This film is catchy, surprising and a rare stylish one on the atomic zombie thematic. I'm a fan of Christiano Galbo too, a beauty but also a solid presence on screen.
One of the most beautifully realized and engrossing zombie films of the 70s. Becomes more rich with each successive viewing; the clothes, the cinematography, the setting and Jorge Grau's eye for finding horror in unconventional places all make this one essential viewing.