There is a feng shui cave behind the house.

From your description, the main problems are that there is no backer behind, the foundation is unstable and the population is unfavorable.

You can put a boulder or rockery in the middle of the wall of the house sitting on the mountain (that is, crouching position) to create an artificial backer and protect the health of your family.

Because the back you are talking about may be the back of the building, the sitting direction of your home should be determined by the direction of your own door, not by the whole building, so this back may not be the back of your home. You only live in one unit in the building, without looking at the layout of the whole building.

If you can see the open space from the window, you can put green plants on the windowsill to cover it, because it depends on the environment and shape, not on the open space, mainly on whether there is a bad shape and whether there are defects when sitting up.

It is convenient to provide pictures and sit upright, otherwise it is easy to talk.

I hope it helps you.