The first paragraph goes against everything i want in a open sandbox games. I dont want to HAVE to do anything. i'd rather be able to try and find a way to sneak around kill the guy im looking for and sneak out or just blast through them all depending on how patient im feeling. So aboveground all the way for me. But above ground doesnt mean you dont ahve to run the "gauntlet". if its above ground the boss could be in the middle of the encampment instead of the end of the cave so you still ahve to find a way to the middle. But this way maybe one side is more of a bottleneck with warriors on a killing ground, while another is open area with archers all around, and a third is a mix. It lets you choose which is more conducive to your fighting style, so you can have more fun fighting the way you want to but still fighting the gauntlet. There could also be an underground root into the middle of the aboveground castle, or a backdoor thats hard to get to but loosely gaurded. It opens up all sorts of possibilities besides fighting from point A to point B killing the endboss and wandering back agian.
The first paragraph of my post is a description of what a typical video game or RPG "dungeon" is, and why it's made that way, not necessarily what I'd want to see in future games. Ideally, I'd rather see more options for different types of characters, several possible routes, and some small degree of random variation in what you face to keep it interesting the second and third times through, but that's a lot harder for the developers to set up well. Doing so in an outdoor environment, where you've got to allow for several different approach directions as well as several different sets of skills, becomes exponentially more difficult to manage. It can still be effective in "controlled" circumstances, as in a narrow pass, a forest clearing with heavy undergrowth limiting movement in most directions, or any other situation where you only have a limited number of routes to consider, but having a large hostile encampment with a "boss" or other objective surrounded by varying groups of opponents would be a major problem to create and have function in a manner that's "playable" by anything other than a "stealth" character, while still remaining credible. No sane warrior is going to charge straight into an enemy camp and take on the entire army single-handedly at once, and any game that allowed you as a mere mortal to do so and survive would be a joke.
That's why I mentioned about options such as charging the heavily guarded main staircase or sneaking around looking for a less-watched back stairwell; you've got two entirely different approaches to the problem for two entirely different styles of play, but channelled enough to be "play-balanced" without a monumental amount of development. A skilled enough negotiator should be able to bluff, charm, or bribe his way past most of the guards to get closer to the objective without fighting, and a magic user should have several options both with and beyond those approaches, either by using magic to augment stealth or personality, or by utilizing a combination of levitation and offensive spells to simply bypass most of the main defenses and then incinerate the rest. The "standard" approach to dungeons, where you just fight your way past one "checkpoint" after another, gets really old in a hurry, and OB's usual "back door exit" to almost every sizable dungeon was only a way of avoiding having to backtrack all that way through the carnage you left on your way in.