The swamp was originally founded by Shrek's parents but they had to leave the place when Moar Krabs invaded the place and destroyed everything. Shrek's father died and his mother was sent to the Area 51. Then the swamp passed to Shrek who was a teen and decided to use the swamp to live in Mushroom Kingdom without being molested by other people. Then, Shrek met Donkey and both traveled to rescue Princess Fionna from Ganon. Shrek defeated Ganon, rescued the princess and killed Donkey for being an ass to him. It's unknown how big Shrek's house is because Shrek has killed and raped several victims who went in.