Lancelot said:
I guess it might have been different if the party was 90% rogues, or if the DM was giving us more than a free ride. I puzzle over statements like: "Smart players should be able to take out the fortress easy." I'd counter with: "Smart DMs should be able to turn this place into a death trap".
Oh, it's definitely got a TPK potential. I'm not disputing that. Heck, I'd go so far as to say "dumb DMs should be able to turn this place into a death trap."
If you read the tactics, though, you'll notice a few things.
Several of the orcs are in the tavern, relaxing. If they don't make their Listen checks, they don't come rushing out.
The archers are spread around the top level. There is a thick fog that prevents them from seeing very far - it's spelled out in the adventure that they use their Listen checks almost exclusively. The wyvern can be fooled by a decent disguise, and so can be avoided. The Zhents don't want to fight. Four of the more effective nameless orcs on this level have a 10% per round to turn on each other.
It can be done. Heck, something as straightforward as a
silence cast by the cleric (a tactic mentioned in the adventure) goes a long way towards making it easier.
It can also be a bloodbath. There are
fifty orcs on that first level. Fifty. Five-O. A 4th-level party could easily be wiped out by that many. I'm not an advocate for 'the monsters sit in the room and guard the pie' encounters, but this upper level is set up to be taken out in small chunks. (I'm not advocating the 'blow your spells then rest' routine; I'm advocating the 'sneak attack +
silence + move silently = dead orc warrior 1's' method.)
As always, YMMV. The OP has a party optimized for stealth and sneak attacks - I wouldn't say a group of fighters if plate can go this route - and that's why I've been giving the input that I have.