One of the things that Ian and I kept coming back to when we were discussing a return to Undermountain for 4th Edition was that you can’t rest whenever the fuck you want in a dungeon like this. There are entirely too many things that would like to eat you, and part of the Undermountain experience is learning to deal with some resource management. You bust a daily in your first encounter, there’s no getting that back for a good long while. If you’re out of Healing Surges and you don’t have anything that heals without using them, you’re in a pretty rough position. Being able to sit down for four to eight hours every once in a while is a luxury, and it’s one you can’t have when running around this dungeon.
Think of Undermountain like a city; it’s a city in which every single one of the citizens would be more than happy to spill your entrails on the ground and bathe in them. Every place in the dungeon gets used, and gets used fairly regularly. The only places that people might visit less often are places that are completely barricaded, places that are only accessible through secret doors, or places that are so incredibly dangerous to get to that no one ever bothers. Those places are, to be honest, pretty few and far between.
Halaster’s a dick. He’s like this retarded five year old with a +5 rocket launcher of doom and a crack habit. He built this dungeon as the ultimate expression of his douchebaggery, and he knows that people are going to come in and try and fuck with his shit. To do that, they’ll need resources, and they’ll need chances to rest up, heal, get their shit in a group, and come up with a plan for how they’re going to tackle the next ring of this circus of death. He isn’t going to make that easy. If you want to sleep in Undermountain, you’d best be prepared to work for it.
Barricading a room seems like it might be the best way to make that happen at first, until you remember how much noise that makes. Monsters like noise, it’s like a dinner-bell to them down here. I don’t have a Wandering Monster chart kicking around for Undermountain yet, but if your players want to barricade more than one door of the dungeon, I’d suggest you kick in one of those doors and make them fight some monsters. “Oy, right, yeah. Heard ya bangin’ about up here an’ thought, ‘Well them’ll make a nice bit o’ supper, ne?’”
So we have a few rooms kicking around that are inherently difficult to find and/or get into. Those rooms have one entry and exit point, and that entry point is easy to barricade and make relatively safe. There's some room for discussion in how many safe rooms there are. Personally, I think it should be every six to eight encounters. So you won't see me detail a Safe Room on this blog for at least three more encounters (probably closer to five). The nature of each of the safe rooms will be different, as well. Some of them won't allow you to sleep; you can meditate fine, if you're an Elf or a Warforged or whatever, but characters that require sleep will find the safe room entirely too uncomfortable for that. Other rooms will make it impossible to mediate, but sleeping creatures will do just fine (some sort of psychic interferance). Some will allow you to do both. Some will have to be barricaded, some will require a skill challenge or an encounter to get rid of the Things Here Before You, some will need to be made more comfortable, some will have tricky little bits that will fuck with your day, some will be completely harmless but have strange features that make you think something's up. It'll be fun coming up with them.
Also, getting BACK to a safe room should be a terrifying ordeal in it's own right. Comfortable rooms in Undermountain are a premium; monsters fucking LOVE comfortable rooms. When you come back to a Safe Room after a couple of encounters, expect it to have become the home of something Terrible and Weird that will need to be defeated before you can sleep away the last three encounters.