When the spectators flee from the arena, one ticket booth is knocked over, leaving the open side facing up. In later shots a closed side faces up, and still later this same booth disappears completely.
Early on, young Lope tells Tuck Kirby that the arena is two miles outside of the town. This is confirmed by Tuck encountering Professor Bromley's paleontological digs in open desert during his return from his first visit to it. However, at the film's climax the arena is clearly within the town.