Castle in a Nutshell, or "What IS the Old Haunt?"

In case you are very new to Castle: Kate Beckett is a hard core NYC police detective. Rick Castle is a rich successful mystery writer who follows her around for "inspiration" - for his book series based on her. If you're wondering WHAT exactly is The Old Haunt, here's a clip of Rick explaining it all - not to mention a great example of what the whole Castle&Beckett 'thing' is about.
https://www.youtube.com/watch?v=TsxNuvbrCUM&feature=related[u]