Challenges in Real Estate Partnership

I understood like this: the activity is private or not depends on one main thing: the activity is made on a person's private land or not;
If it is true, from the rule, in the case of shared courtyard, when both neighbours agreed to define and divide the shared land they have to build a wall, I derive that visual trespass is a trespass, because now the activities done in a private land are protected from being observed by a law (they are obligated to build a wall).
Could it be a valid opinion? Reply