Ok. I died on I40. Had some trouble in previous levels because of lack of food (even while using the cloak sparingly): had to eat a kobold corpse to avoid dying at some point. Got healed a few levels later by a random healing potion. The down staircase in I40 was in a tension room of chaos wizards (yay). When I found out I was surrounded by chaos servants. Had some fun hitting them in darkness but at some point they were replaced by a karmic lizard (got cursed) and then writhing masses of primal chaos, which took me down slowly.
My Drakeling was level 12 when facing this poor end
How about this: you can only visit any level twice, at most. This is actually the only rule needed, as anyone who wants to play it safe in regards to rivers can choose to not go up until he or she comes across a river blocking the down stairs. It's very unlikely to get two river-blocked stairs in a row. On the other hand, someone who wants to maximize loot and xp, might have to try crossing a river.