The cops (who weren't really that mad) told me that anti-climb paint is kind of like pigeon glue; it has a long-lasting viscosity that makes surfaces sticky, slippery, and just plain unpleasant to perch on. It also sticks to human skin, so back in the day you'd be able to identify burglary attempts by checking the suspects' fingers. But, they said, "it might not be working anymore"; it's been a long time since people actually tried to break into Oxford Castle, which is now a tourist destination.
I'm a contributing editor here at Boing Boing. I also have a blog (TokyoMango), a book (Urawaza), and I freelance for Wired, Make, the NY Times Magazine, PRI's Studio360, etc. I'm @tokyomango on Twitter.