The billboard Castiel crashes into in the beginning of the episode, is an advertisement for an attraction called Mystery Spot which is from an episode(11) in season 3 that revolves around that attraction.
This is the first time the show has actually shown us what happens to an angel after they are banished.
The title Keep Calm and Carry On is from a motivational poster produced by the UK govt during the Second World War. This is now used as a slogan on various goods such as mugs and T-Shirts, with 'various' adaptations aplenty.
Lady Bevell uses a form of Chinese water torture on Sam, which actually was invented in Italy in the 15th century, how it came to be associated with China is not known for certain. The process involves dripping cold water on the scalp or forehead of a person for a prolonged period of time, it is more of a mental torture than a physical one. It does cause some physical pain such as headaches from the cold water, but mostly it is meant to cause anxiety and panic, as it can cause a person to fear drowning. The most effective form of this torture is not to use a steady stream of water like Bevell did, but to use drops that fall at random intervals. The pattern of the drops is often irregular, and the cold sensation jarring, which causes anxiety as a person tries to anticipate the next drip, if the drops are in a steady pattern the person can learn to anticipate them and get used to it, but having them randomized has been shown to be far more effective. It causes fear and mental deterioration and can actually cause lasting mental damage in some people, in one recorded instance a person being tortured with the random drop method had a total psychotic break after only 20 hours.
Starting with this episode, Andrew Dabb and Robert Singer are the final showrunners of the series.