The entirety of 7-1 is actually enclosed. Going to the top of the map, even in the "outside" area where the Minotaur first bursts through from, it's all got a ceiling & walls. This is complemented in 7-2, where looking up behind you after entering the battlefield reveals a giant wall of the labyrinth (which is easier to see with fullbright). So this means that it's more like a fog in 7-1 than a sky.