Over on his excellent (mostly Car War oriented) blog, Jeffro posed some interesting questions about B2 The Keep on the Borderlands.
Keep on the Borderlands: Things Gary Gygax Never Told You.
I decided to take a crack at answering them. These answers are not 100% complete because I had to omit things my players don't know yet. They are all campaign-specific to me. These are not the "right" answers, only right for my game. Feel free to plunder them yourself.
Why are there so many monsters of differing types in the Caves of Chaos?
Answer that and you need to answer the question of why dungeons are loaded with many different monster types and, heck, why the rainforest is filled with hundreds upon hundreds of different types of critters. It's biodiversity. ;)
For my game, there is a better question - why hasn't one group dominated the caves?
These aren't apex predators. These aren't highly organized, large groups of humanoids. Like the folks at the fort (see below), this is the frontier. The highly organized, large groups of humanoids don't sit on the borderlands. They sit in the heartlands - the trackless wastes, the Misty Mountains, the depths of Moria, the benighted realms of the far north. They aren't crammed into the shallow caves near the human lands.
So who is there?
The losers, the outcasts, the fresh young would-be chieftains and their followers, the shattered remnants of bigger tribes, the pocket tribes of ill-organized goblinoids. You're not dealing with the Great Hobgoblin King here. You're dealing with tribes not strong enough to thrive in the heartlands, and willing (or forced) to risk the edges of human civilization. Some are there to make it big, some out of desperation. They're the mirror of the humans who come to raid them. They're greedy, murderous kidnappers. Murder Homeowners instead of Murder Hoboes, to quote sir pudding over on the SJG forums. They don't wipe each other out because they aren't powerful enough to do it, and none can trust the others to really help.
Why doesn’t the Castellan just send a detachment down to the Caves and clean them out?
My keep was built in response to a much earlier threat. Ten to twenty years ago, there was a big problem in this area. It was "cleared" and the threat was contained with a spiffy new fortress that was constructed. Now it's completed, but the threat has long since faded from the forefront of concerns. Wars elsewhere have distracted folks, etc. So the keep is there, but it's barely funded and it's a backwater posting. You don't get posted to the keep because you are elite, but because you just happen to be handy when they need someone and haven't shown yourself valuable enough for private service.
If the Castellan clears the place with a "detachment" that's great, but he can't replace losses or afford much danger pay. If it suffers defeat, he's screwed. Try explaining that - "I launched an expedition to raid the Caves of Chaos, which barely had enough goblinoids in it to bother us, and it suffered a TPK. Can I have some more guards?"
If he just leaves it alone, and adventurers happen to show up and raid the place and he lets them base out of the Keep, well, that's even better. It's a frontier fort, and replacements aren't handy, and it's easier to wait for a tilt in the balance of power than to try to force one. Plus, inevitably, he's got other duties - patrolling that road, keeping the civilized areas nearby patrolled and safe, etc., not expanding the empire.
How can low level characters hope to make a difference if the high level NPC's are unable to do anything?
What "high level" NPCs? In my DF game at least, the 250-point PCs aren't the top of the food chain but they aren't the bottom, either. But the real ass-kicking hero types, if they are out there, aren't going to spend a lot of time clearing the caves for the paltry loot within. And the world isn't littered with high-level NPCs like the Forgotten Realms or other published settings often are.
Even if that was an option, the NPCs are all busy - see the answers above. The Castellan? He's no major league asskicker with clout and an army. And what if he runs into something he can't handle and leaves the borderlands unprotected? Not a good idea.
Why does the module include precise details of the defenses and strength of the Keep itself?
In my opinion, it's a base and a model. A well-detailed base means you're ready for anything from errant thievery by the PCs, to monster attack, to modeling your own PC's fortifications. It also makes it pretty clear that the base is safe - lots of heavy weaponry and armed guards means the goblins aren't going to chase you home and massacre you while you sleep. So you can run a simple game of "raid the Caves, go back to safety and greet new adventurers to replace your casualties." I've seen the "the Keep is the target" argument and it's fun, but Gary Gygax also wrote out every stupid detail of Hommlet and my players spent a grand total of 20 minutes there in decades of gaming and at least a dozen run-throughs of the moathouse. Why? I dunno, why did Gary write up minutia about stuff people don't care about? Because he did.
And I appreciate it here, because now when my players ask me what the Castellan is wearing or who lives in the apartment marked 7b or what was in someone's pockets, it's all written out. And I have a handy keep just in case I need it for anything. Thanks for that, Gary.
By the way, Jeffro is running his kids and his friend through B2, using the original rules it came with. It's really entertaining and if you think my DFers nailing gnoll hides to the wall at a rate of two per second isn't Old School enough, maybe running Moldvay is. Great stuff and it's fun to see his kids get into Basic D&D the same way I did.