Unbeknownst to them, there is a nasty lich behind it all, building an undead army. Well, they got up the mountain today and made a bit too much noise and so the zombie lord inhabiting the upper level of the castle ambushed the PCs with a shitload of zombies, ghouls, and ju-jus. Turning failed due to the evil power of the castle area (the lich has opened a gate to the Negative Material Plane to empower his army) and so they had to retreat. God, was it fun watching them sweat out an almost certain death. Got out by the skin of their teeth. Quite a shock since they've had it relatively easy so far in the adventure.
It really sucks to have 18 ghouls, 11 zombies, and 8 ju-ju zombies led by a zombie lord facing you, laughing at your attempt to turn them, and realizing you've been ambushed and surrounded. Had the cleric not made his saving throw vs. paralyzation from the ghouls, it might have been a TPK.
