Pokey, you do have to close it, as it's part of the main quest. But you can always go back. It's something to do with those levers that control the big gates outside. I was getting frustrated at not being able to go up any higher in the central tower but went back into the main tower, and the next door I tried was the right one, because there was a hallway with a bend I hadn't noticed, and up I went. Who knows why it was so hard to find? Problem solved, anyway.
To ensure that I don't kill anyone important in a cave, I sneak up behind them first, find out whether or not they have a name, and if not -- well you can guess the rest. If I were a mage apprentice or a necromancer, and knew that an uber-assasin was on the loose, I'd stick a post-it note on my back with my name on it.
Ah, the advantages of having sneaked virtually all the way through the game. Although it takes some of the challenge out when nothing can see you. I've never appreciated the backstab so much! Especially with a nice little dab of poison on the tip of Sufferthorn.
Keep them mowing blades sharp...