This is the latest in a long series of shelter/enclosure pieces. Not unlike the settlement works it seeks to portray an enclosure that is safe; a place to be. Union relates to the spiritual realm and not the geophysical.
In a world of independent thinkers I rebel, like the ancient warrior/shepherd David. When writing his psalms he often describes a personal safe place: relying on his God. This picture, Union, could be a spiritual retreat, a personal safe place.