Oofta
Legend
Bad guys are generally reasonably intelligent. It's highly unlikely the BBEG will have a inner sanctum that does not have an escape route or is virtually impenetrable barring a siege. If the BBEG is an undead, I hope the PCs are ready to wait it out a long, long time.
So camp outside the doorstep and the BBEG knows you're there or can reasonably deduce it because all the patrols don't report in? At best when you do go in the place has been cleaned out. If you're unlucky, the place has a lot of traps in addition to being cleaned out. Worst case they call for reinforcements and when you wake up (if they don't just attack while you try to sleep) you wake up to find a small army ready to discuss the conditions of your surrender.
So camp outside the doorstep and the BBEG knows you're there or can reasonably deduce it because all the patrols don't report in? At best when you do go in the place has been cleaned out. If you're unlucky, the place has a lot of traps in addition to being cleaned out. Worst case they call for reinforcements and when you wake up (if they don't just attack while you try to sleep) you wake up to find a small army ready to discuss the conditions of your surrender.