The Dojo of Club Penguin (CP) is a room located in the mountains at the top of the map which dominated the classical age of Club Penguin armies. The old stories of the great days of Mammoth, the "war server", all centre around the original version of this room. Everyday penguins would log on and flock to the Dojo on Mammoth to join in massive battles. One didn't even have to know about army websites to find out when wars were on - they could always count on having a fun time on Mammoth.
In those days, ninjas lived only in rumour, and Card Jitsu was not even imagined. It was CP armies that made the Mammoth, and the Dojo on Mammoth, the #1 place to be in all CP.