Astonishing and inspirational is the fact that the game has been in development for 15+ years and still is very well architected and designed to the point that it can be ported with such ease.
They did the smart thing and all of the game core is a separate C# package and they were using Unity mostly for the display bit.
Zero game logic was trapped behind unity, so it was mostly just replacing a few idiosyncrasies between Unity and Godot (X vs x in coordinates and colors being floats vs ints)
Zero game logic was trapped behind unity, so it was mostly just replacing a few idiosyncrasies between Unity and Godot (X vs x in coordinates and colors being floats vs ints)