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/