When Dagineau lifts the dead legionnaire Fouchet onto the ramparts, he is able to easily do it using just one arm, revealing it to be a mere dummy not a real actor (notice the cut to the Geste brothers in between to disguise the switch).
Throughout the film the legionnaires wear the collar insignia of the 2nd Regiment of the Foreign Legion (2e REI.) Yet most of their geographic references are to Algeria. When the detachment relieves Ft. Zinderneuf the previous commander's orders are to return to Sidi bel Abbes, the Legion HQ in Algeria. Likewise, during the mutiny the legionnaires discuss escaping across the border to Morocco. Additionally the legionnaires are in combat with the Tuaregs, a Saharan tribe found in Southern Algeria. However, during the period of the film (and throughout the inter-war period) the 2e REI was stationed in Morocco, fighting the Berbers, and not in Algeria, which was instead garrisoned by the 1e RE.
They have a machine gun and also a cannon which can fire shrapnel - real potential "difference makers" in their besieged position. They indeed place the machine gun very high in the fort (until it runs out of ammo) - but uselessly place the cannon at ground level inside the fort - where it can only be fired by quickly opening and then quickly closing the fort big gates - which they only do once anyway.