There is, of course, a third factor, and that's luck. Just a few nights ago I was trucking along nicely with a 9th level character, only to be surprised by an Evil High Priest and killed before I could even react. It happens; sometimes the random chance goes against you. But I've gotten to a point where I feel like I've minimised that random chance as much as I can. I'm starting to encounter 10th-level monsters now: Dragons, Balrogs, and even the dreaded Programmers. With smart spell usage and a bit of luck I find that I can take them out without to much trouble, and if I do find myself overwhelmed I can always escape with a Teleport. Hopefully next week I'll be able to make my final post on the game, and move on to something else.
Before I finish this post, I did want to point out how awesome the Arrows of Slaying are now that I've found some. I found a bunch of ten and a bunch of eight, and they're indispensable on the lower levels of the dungeon. If you have any of these arrows, you get the option of firing one whenever you encounter a creature of the Mythical class, and it will kill the monster outright. There are some nasty monsters in that class: Balrogs, Dragons, Griffins, Lammasu, Purple Worms, Umber Hulks and Wyverns make up a sizable portion of the level 9 and 10 monsters. Having that instant kill in the arsenal is a life-saver.
This was a short post, so here: have some more maps.