I thought about simulacra. That's an illusion spell. Wouldn't true seeing see through it?
My current idea is that the evil mages do have sleeper agents - a number of them, including one aboard the PCs' ship. Just before the ship arrives, the evil mages' sleeper agents de-activate the dimensional locks in the castle, the mages teleport in and grab the Queen's husband and baby daughter. The Queen's crown, orb and scepter are magical artifacts that hold the key to the kingdom. The person who wears the crown controls the island's movement and so forth. You have to be good to wear it without suffering negative levels, but the wizards don't know that. They threaten to kill the Queen's family unless she co-operates by doing exactly as the cabal says. Now the Queen isn't stupid, and she's a powerful mage, and not likely to take this lying down. I figure, however, she decides to pick her moment carefully and so does nothing.
Enter the PCs. Their ship arrives and to get them onto the island, the Queen, being directed by her captors, invites the senior crew to dinner. This includes the PCs, including one who is a wizard who escaped from the cabal years ago and now works against them. While the PCs are away, our sleeper agent sneaks off the ship and onto the island.
The PCs will be able to tell something's wrong but not know what. While they're in the castle, the cabal places it in 'lockdown' mode - dimensional locks, walls of force, arcane locks and so on - nothing gets in or out. The PCs are now prisoners in the castle. The cabal wants the PC wizard to return to the fold, and lock the others up so he'll co-operate. Meanwhile, the sleeper NPC shows up, saying he is to be the new puppet ruler of the island. The Queen knows what will happen if he wears the crown, and by putting it on his head the new 'King' goes insane and gains negative levels. This doesn't concern the cabal, so long as he does as he's told.
The PCs therefore, have work to do. To save the day they have to take on each of the cabal, or defeat them some other way. They have to locate and secure the Prince and Princess so the cabal has no bargaining power. One of them has to resist coercion into rejoining the cabal, the others have to escape from their forcecage cell with an antimagic field around it. I also like the idea of the NPC 'king' actually starting to feel guilty about betraying them - the crown has made his mind go gooey but he still feels responsible. He offers to let them out if they take him back - they're unlikely to do this but it's good character stuff. I figure he could be the redeemed bad guy who gets killed by the cabal before the end of the adventure.
What do people think?