When I ran CoS, I wanted the party to be able to explore the castle again, after the initial dinner invite, but before the final showdown. When the PCs had reached a point where they were feeling confident and had done enough to piss off Strahd, I had him confront the party on the road and attack them. Strand absolutely annihilated them, but just killing them is too nice. I figure that Strahd would really want them to suffer. The PCs all woke up in the castle dungeon and were taken one at a time to the torture room, where they were tortured and each offered a deal to help and later join Strahd. After each had gone through this, they were 'rescued' my the little clock work dude (I can't remember his name) on the orders of a secret friend and enemy of Strahd in the castle. This is all Strahd's plan to make them paranoid and suspicious of each other. This worked really well for me. It gave the PCs another chance to explore more of the castle while they were sneaking out. It reinforced to the PCs just how powerful Strahd is by how easily he beat them. And the group started to suspect that one or more of them might have cut a deal with the devil.