Imagine this... you're peacefully minding your own business (and slaves), in your isolated and hidden fortress, you've got an amiable agreement with your local hobgoblin clan, and the hordes of ravenous undead keep most everyone else away. The current crew of hire-swords seem to be working out, you still have a moderate number of slaves healthy enough to work, and your research and training seem to be progressing decently....
When out of the blue 3 lantern archerons come by, destroy the slave you left out as an example to others (undead after a wight wandered by, struggling feebly in his chains, what better example than that from a slave too weak to work?) then make a nuisance of themselves until you and your fellow brothers blast them out of existence.
Archerons don't wander this mortal plane without reason, and when you look outside, you see 3 or 4 capable looking men (some in armor, 1 fairly injured), and a hill giant (also bearing injuries) escorting over a dozen people in rags with makeshift clothes and various ill-fitting hobgoblin gear, just over a hundred yards away. The capable ones in the bunch seem to argue a bit and gesture directly at where you are (even though your fortress is hidden by magic, and you regret not memorizing Scrying, eavesdropping would be so nice right now)... then the whole bunch starts to move on with the apparent intention of not leaving any slaves to replace the example they deprived you of. Oh.. and leading them is a pretty bloody hobgoblin that seems to be afraid of every little shadow and flinches at every sound it hears.
So.. what would you do?
This is where our last session left off, and while I did have a plotline that involved the PCs sneaking into the fortress, one made a mistake earlier that let the mooks overwhelm him, so the party is a bit more beat up than planned and decided to return later to deal with this. While I have an alternate course of action planned (which I won't mention as at least 1 player reads these forums), I'd love to learn how other DMs would continue this.
What you can do is this -
Most of your forces hide, with a few guards defend and pretend to go unconcious once attacked several times.
Once the group of attackers is inside your keep, hit them on multiple sides with Black Tentacles, Confusion and Ray of Enfeeblement. This should enable them to be captured. Can throw in some tanglefoot bags for good measure.
If they need a lifeline to get out, perhaps the Hobgoblins and/or the Hire-swords are sick of their employer and want allies to take over this place.
Also one of the slaves could be cleric or druid who can heal PC's if sufficent diversion is created.
Why capture them? well the main baddy needs slaves or maybe just someone to talk to. Music or fine tales may gain the PC's favour.
For a twist the main baddy could capture the PC's, then ask for help. You see this powerful undead guy rocks up once a month and demands a few slaves and checks how the undead troops here are progressing, threatens the main baddy to animate him if more slaves are not gathered...
Another twist is the main baddy is gathering forces because he used to belong to a guild of nasty wizards, and some of them have setup bases in the area. The baddy may be the thing that is stalling a major incursion of planar beasts like slaad or githyanki?