For a while, a few months ago, I was going around my workplace with a sock puppet. A crude, foul-mouthed sock puppet. The coworkers loved the sock puppet, but one day I cleaned up a spill with the puppet and I had to throw him away. It was a terrible tragedy.
Anyhow, one day during this period I went to the men's room where I work and someone in there was making a horrifying stink. It was paint-peelingly bad. So when I went back to the office the sock puppet had to comment on it.
"I just went to the bathroom," the sock puppet told my coworkers, "and it smells like someone's building a shit castle in there!"
For some reason, this turn of phrase stuck. Now, whenever someone goes to the bathroom, they're likely to say, "I gotta go build a castle."
If we can't find someone, someone else will invariably suggest that the missing person must be "building a castle."
Funny how little phrases get into the language, isn't it? I think you should start saying it, too.