Who says there isnt more, though? There were teleporters from room to room, there could be more to the tower which we havent seen. There's probably a large teleporter that would take them to the moon, somewhere where the team doesnt explore.

If Golbez already had control of the tower, he could have used the tower to get the Red Wings down. The tower has to open to let the giant out, maybe there's large docks to let the airships in and send them down. He could have had agents working down there even while he's gathering the last of the Light crystals.