Doh!!
Ki Ryn said:
So these evil folks use slaves eh? Where did the slaves come from? They were probably the last group of fools to sneak up to the door and try a knock spell
...*snip*...You just need to think about why the bad guys would want them alive (information if nothing else), and what they would do with the prisoners once they have them. Nothing is easier to escape than an ad-libbed prison, so make it real and figure it out before hand. It can still be a great adventure.
I can't believe I left this out. I know it was late when I started the thread, but still...
The slave that the party rescued was in fact a member of an adventuring party that had been hired (by someone other than the sage that employs the PC's party) to basically do the same thing the PC's party is supposed to be doing. They didn't have as much info to go on, however. They strolled into the ruins bold as brass, ran across a patrol of the creatures inhabiting the ruins, and jumped them. The fight went pretty well, except that one of the creatures ran away.
While the NPC party was busy patting themselves on the back for kicking butt and driving off the 'cowardly' foe, "The whole world crashed down on us," said the slave. They were counter-attacked by a large organized force of the creatures, had their butts handed to them on a platter, and found themselves enslaved. They were sentenced to five years hard labor for the crime of trespassing on forbidden ground, and for suspected thievery. Iosef, the rescued slave, believed that while the creatures may well honor the sentence and free them after five years, he really doubts they'd live that long.
The creatures used them as 'trap-springers' in the best Wulf Ratbane tradition.

"You, slave! Walk down that corridor and see if anything kills you, or we'll kill you where you stand." Since then one of their party has died from springing a particularly nasty trap, and they have all been nearly killed a few times. The creatures keep patching them together so as to get the most use out of them.
So that's one of the bits of info the party has available to them. Sorry again for leaving that out - it explains why I got so many suggestions to show the party an object lesson demonstrating the creature's abilities. I already have done so, but didn't mention it. *drat*drat*drat*