Definately the dungeons at Warwick Castle. Went there when I was about 12, and was quite excited, I wanted to see the dungeons (even though I've always been a bit squeamish and not really into that kind of 'yucky' stuff like most kids). But as soon as I got down there I had to leave. The smell, the damp walls, the equipment on display. It left me feeling very low, miserable and pretty ill for the rest of the day. Even now after 15 years, the smell of cold, damp stonework can bring back that ill, sickening feeling. No idea if its PL related, or just me picking up on the residual emotions of all the people who met such nasty ends down there.