Mario's Castle was (and perhaps still is) the plumber's most grandiose residence. A fanciful luxury home rather than a functional fortress (in the vein of the 'follies' of Bavaria), it was situated in the center of a large island apparently also privately owned by Mario, hosting various themed areas and an enormous mechanical incarnation of the mustachioed one. Quite how Mario came to acquire this land, not to mention the castle, is open to interpretation - a popular theory is that it was the manifestation of his tendency to horde coins, and that he was spending them on more than extra lives and bonus items.
When Mario journeyed to the far-away kingdom of Sarasaland to save Princess Daisy from Tatanga in Super Mario Land, his grotesque alter-ego Wario seized the castle in his absence, with the motivation apparently being jealousy. Once in control, Wario palmed off the 6 golden coins required to gain entry to the castle to his 'friends' at the time. In addition, he decorated the interior with enormous spiked balls and bubbling pits of an unspecified lethal liquid. Of course, Mario eventually reclaimed his castle from the archfiend. Wario was later to obtain a castle of his own in Wario Land: Super Mario Land 3.