I personally imagine that it’s a mix of penance and hope for the future. From Ozma’s perspective that castle (as well as where Salem grew up) were meant to be homes and both spaces failed spectacularly at that. He saves a young girl from a tower. He’s later re-united with the love of his life, has a bunch of kids, finds purpose in protecting the people as their new god… and then it all came crashing down around him. Beacon is basically the castle 2.0, a space where he’s determined to do things right this time. Now it’s the primary defense against Salem—both in training huntsmen and in protecting a relic—rather than a way for her to consolidate more power. Now it’s filled with much older children who, rather than being vulnerable and at the mercy of the one person meant to protect them (their mother), can protect themselves and are able to trust in the protection of the adults around them. Now instead of ruling supreme Ozpin shares the load with his inner circle and presents himself not as a god, but as a fallible man just trying to pass on knowledge. He’s reversed everything the castle once was.

Ozpin loves moving forward, but he also loves self-punishment. What better way to do both then by turning your school into an updated version of the home you once lost—or rather, the home you only deluded yourself into thinking you had? You were once so arrogant you put yourself above everyone else? Do it literally now. Metaphorically lock yourself up in Beacon’s tower like Salem was once locked in hers and remember that you can never repeat those mistakes.