Castle Anthrax is a fictional stone-built 8th century fortress a few days ride from Camelot visited by Sir Galahad the Chaste (Michael Palin) in the movie Monty Python and the Holy Grail.

All residents of the castle were female. The castle was unusually well furnished, containing big beds that were both warm and soft. It was also distinguished by a grail-shaped beacon that was visible to approaching travellers.

The community comprises "eight score young blondes and brunettes, all between the ages of sixteen and nineteen-and-a-half."

Galahad first meets Zoot (Carol Cleveland), one of the castle's commanders. Their time was, according to Zoot, generally spent bathing, dressing, undressing, and making exciting underwear.

Galahad tells the women that he is there seeking the Holy Grail which he has seen above the castle. Zoot's identical twin sister, Dingo (Cleveland), then realizes that Zoot has been setting alight their grail-shaped beacon. She explains that the only punishment for setting alight the grail-shaped beacon was that the guilty party is to be tied down on a bed and spanked. The rest of the girls offer to be punished by Galahad, to be followed by oral sex.

Sir Galahad is invited to inflict the appropriate penalties, but Sir Lancelot and company burst in to "rescue" him, professing the terrible peril he was in. Galahad dutifully volunteers to stay at the castle, explaining, "Look, it's my duty as a knight to sample as much peril as I can", and as he is dragged out of the castle exclaims, "Look, I can tackle this lot single-handed! Honestly, I can cope. I can handle this lot easily. There's only 160 of them!" The women all say, "Yes, let him handle us easily!" As they escape, a visibly disappointed Galahad says to Lancelot, "I bet you're gay"; Lancelot pauses before indignantly replying, "No, I'm not."

In reality, most of the many rooms in the castle were the same room, the kitchen in Doune castle shot from numerous angles. Doune castle also masqueraded as several other castles in the movie. both exteriors and interiors.