Defender is an ancient game that presents a virtual world, a horizontal scroller, in a loop. The camera view only portrays a portion of the world at any given time. The player can fly in both directions around this world, continuously.
How could this be achieved with an SKTileMapNode Layout of the world?
My first thought is to have two instances of a SKTileMapNode that illustrates the whole world, and abut them in space where and when required as the player flies around the world.
Here's a map of the Defender world highlighted at the top of the screen: