The Dojo (before being struck by lightning/renovated for Card-Jitsu) was a secret location in Club Penguin hidden on the old map in the mountains. It was built long before penguins inhabited the island we now call “Club Penguin”.
You couldn’t do anything in the Dojo before Card-Jitsu so it was mainly used for parties and snowball fights.
Since Dojo’s are related to Ninja’s, many Ninja theories arose (there were even Ninja Shadows on Club Penguin before Card-Jitsu).
There was a rumor that if players stand completely still at the Dojo for 30 minutes, you would become a Ninja. Aunt Arctic confirmed that this wasn’t true since your penguin would be logged off automatically if it were idle for more than 10 minutes.