There is no evidence that the founders actually built Hogwarts Castle, and the castle was not purpose built to house a school (Hogwarts: the Castle). However we do know from the sorting hat that one founder did build something that would become incorporated into the modern day castle: the Chamber of Secrets.
The myths surrounding the Chamber of Secrets are contradictory and nonsensical, just like the information we have about Slytherin himself. Although legends say Slytherin left a monster to purge the school of muggleborns, the basilisk itself is an indiscriminent weapon and harmful to the entire school. When it was unleashed Hogwarts was nearly closed down twice. Even if we believe all that is said about Slytherin, there is no doubt that he loved the school he had helped create. Therefore it is more likely that the Chamber of Secrets and the basilisk within was placed there for an entirely different reason.
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