That structure in the left of the image is a generated structure in the world. It is randomly located in the world, usually far above the ground, and its portals connect to 4 other towers generated relative to it. In the flat world type, where I do most testing and most of these videos, it generates right at the world origin.

In the next release, I will need to come up with something similar to a generated village since I want to introduce something like a villager entity for trading. Exactly how I want their behaviour and trades to work will still take some thought, though.