Hmmm - well, the rule says there has to be a grid - doesn't say it has to cover the whole map sheet. Say your hapless campers are well and truly stuck on the lower level... "climbing the cliff succeeds on 4 or less, on a 5d10 roll.... sorry, FAILED." Agreed the vivid, sharp grid looks off, but how about a subtle gridding of just the cavern floor, and MaYbE the terrain just outside?