Oddly, the middle levels of a dungeon are the most dangerous. Why? Due to the bug in how damage is dealt, the average damage on Dungeon Levels 5–6 is quite high, higher than the levels below. Fortunately the monsters here don't have any nasty abilities. The worst is the Mimic, which appears to be a chest. You may want to Attack all chests on sight, just in case. Just keep an eye on your HP on these levels.
Dungeon Levels 5–6 lack any particularly annoying Creatures, with the only "special ability" among the lot being the Mimic's ability to appear to be a chest. You may want to Attack all chests on sight, just in case.