The troll says "Hmm... very pertinant questions, Arkesh.
If I were on the other side, I would first use all means at my disposal of gathering information about the defenses and the resistance I'd meet. Having already used Divinations myself to try and learn the nature of our attackers, I can tell you that they probably have vague information about us at best, as that's all I have been able to gather about them, despite a great deal of effort on my part. I know that among them are a powerful dragon, a wizard, and a warrior who prefers to fight unarmed, and that their attack is eminent, though more than that I can not ascertain. Given our... diversity... their information is probably even of poorer quality.
Information about the compound is probably more readily available, as it has a history that could be researched using more mundane methods. So we can expect them to know the layout about as well as we do.
As for how I'd approach it... hmm... that is rather trickier. It would depend on my objective. If it were merely freeing the current slaves, I'd want to think more like a thief, and avoid detection and conflict as much as possible. If I wanted to make operations impossible here in the future, I'd want to somehow destroy this place, or make it unusable, but that second part would be rather difficult. So I'd guess that they're going to try and pluck the slaves from us under cover of stealth and misdirection. At least... that's what I would do.
I thought the barracks would be a better place to house the slaves due to the fact that any attackers would have to pass through either open air or water to get to it, since it is an island. They'd also have to pass through one of the two entrances to the chamber, allowing us to focus our defenses there. Of course, I see your point about the barracks chamber still being a rather large area... If we have a better option, we should use it."
Looking at the map, he continues: "Maybe this smaller chamber to the east of the barracks? We could probably afford to cover the back of it with Forbiddance, and maybe Private Sanctum.
Or maybe we could use the idea suggested earlier about putting the slaves inside extradimensional spaces like Rope Tricks. A single casting of Magnificent Mansion could house over half of them, I think.
Oh! Or we could combine those two ideas, and use this smaller chamber as the place to mount our defense, while placing the slaves inside an extradimensional space which opens into it, so we can keep a close eye on them, but still have them kept safe. We then layer this area with protections against divination and dimensional travel, and just fill the rest of the compound with as many traps, poisons, and monsters as we can muster before the attack? Then we wait, and prepare ourselves for their assault. They'll muddle through a trap-filled mess until they find their way to us, where we finish them off.
We could launch short attacks against them and fall back to here while they're trying to penetrate into the rest of the complex, also.
And finally, I was thinking of calling some lantern archons to use as scouts. They can teleport around and report sightings of invaders. I think I can convince them that we're the "good guys" here, at least long enough to get some use out of them."
