The source files are freely available for non-commercial use.

Access works similarly to Epic’s Unreal Engine model. Those with a valid game licence, who accept the programme terms, will receive an invite to Hytale’s GitHub organisation where they can view, clone ...

These days, most high-profile games are basically a black box as far as the public is concerned.Slay the Spire 2, however, bucks that trend: and it all comes down to the deck-building game's engine.