The tweet from Cellar Door Games says:
It's been over 10 years since we released Rogue Legacy 1, and in the pursuit of sharing knowledge, we are officially releasing the source code to the public. https://github.com/flibitijibibo/RogueLegacy1/ Eternal thanks to @flibitijibibo for setting this up, and being with us since the beginning.
Here's a direct link to the tweet: https://x.com/CellarDoorGames/status/1846246914406195662
Here's a direct link to the repository: https://github.com/flibitijibibo/RogueLegacy1/
Custom license is not ideal but works for a modding and custom game scene. I would be wary of looking too closely at this before making your own game.