Originally Posted by
Greyfeld85
Chalk it up to a faux pas in DMing. Even if you hadn't taken the time previously to map out any defenses for the BBEGs' lair (under the mistaken assumption that the PCs wouldn't head that way), you should have taken a break to draw up something really quick. Or possibly called the game for the night to give you some time to hammer something out.
You have to keep in mind that you're not just challenging a group of players, you're roleplaying villains. Intelligent villains. Intelligent villains who have mooks and a lair they feel is worth keeping secret. Allowing the players to traipse through their lair without any sort of alarm or pre-warning system, no traps, no mooks making a tactical retreat to set up an ambush... that doesn't sound like an 8th level wizard to me.