In Biggles Foreign Legionnaire, Zebrit was an Algerian town where a Foreign Legion training centre was located. Biggles, Ginger and Marcel Brissac were stationed there for a time after they joined the Legion undercover. Other locations in Zebrit included the "Bar Pigalle", a watering hole for off-duty legionnaires run by Louis and his wife, a grove of date palms beind the bar where secret meetings and illegal transactions were often done, a native quarter or "souk", and the Villa Mimosa, residence of lawyer Jules Raban.
Zebrit is a fictional town. Johns does not specify where it is exactly. It is not Sidi Bel Abbes, because Biggles and Ginger passed through there on the way to Zebrit. In chapter IX, Biggles and Ginger went at midnight to the Villa Mimosa and after some formalities, were sent on a car trip to Algiers, arriving around 5.30 am. This suggests that Zebrit is about 4 to 4.5 hours from Algiers, perhaps 120 to 180 miles away. A real world analogy would be the town of Saida. A regiment of the Legion was stationed there and it is about the same distance from Algiers.