Game

The game class is a starting point for your game code to be called by Engine, and nothing else:

internal class MyGame : Game
{
    public MyGame()
    {

    }

    public override void OnUpdate()
    {

    }

    public override void OnDraw()
    {

    }

    public override void OnDrawGUI()
    {

    }
}

OnUpdate and OnDraw are abstract and must be implemented. OnDrawGUI is virtual and optional — it's called after the main draw loop, outside of any camera transformations, making it ideal for HUD elements and debug text.

It is recommended that the Game class be treated as a scene management system, as opposed to hosting any actual game logic. Most of your game logic will be written into entities. More on this in the scene and entity docs.

SimpleGame

If your game only needs a single scene, SimpleGame wraps it for you:

Engine.Initialize("My Game", 256, 256);
Engine.Start(new MyScene());

Under the hood, Engine.Start(Scene) creates a SimpleGame that calls Update, Draw, and DrawGUI on that scene. You can also pass an IEnumerable<Entity> to Engine.Start, which wraps everything in a SimpleScene and then a SimpleGame.