The Tower of Existence supposes that some kinds of existence (or ontologies) are more fundamental than others. The reasoning goes that you can construct one kind of existence from another, and so it follows that the most basic existence, the one which cannot be constructed upon any other, must then be the foundation of our tower.
Definition Objective (adj.):
Existing as a property of an object, independent of any observer.
Most often, I see Objective Reality used as the foundation for the Tower of Existence. Objective reality is the reality that exists outside of your head, the reality of matter, of light, of the interaction of forces and particles. Objective reality is the realm of the stars burning in the cosmos and the leaves blowing in the wind. All things that exist are made out of objective [1] materials, and consequently it makes a sensible (perhaps even obvious) foundation for our tower to stand on.
[1]"Objective" is the term I see subject to the most abuse. A common example might be the statement "X is objectively good/bad". This implies that goodness and badness are inherent properties of objects. This usually means the speaker is projecting their subjective opinion onto objects, thereby removing themselves as the source of their moral reasoning.
Definition Subjective (adj.):
Existing as a property of an observer in relation to some object.
If we've rooted our tower with a solid foundation on objectivity, we can then place subjectivity on top of it by recognizing that subjects are a kind of object. Humans, for all their ingenuity, are just very complicated molecular machines, the most complicated portion of the machine being the brain inside their heads. It is in that brain that their opinions, ideas, and thoughts form, in the very material firing of neurons and cascading of chains. All of their subjective opinions of the world derive from these simple mechanics, and so ultimately the foundation of subjective reality is based in objective reality.
Definition Interstitial (adj.):
Existing as a subjective property shared across a large number of subjects.
Once our tower includes the concept of subjectivity, it can be extended with what are called Interstitial Realities. This term isn't widely known, so I'll take some time to clarify it.
An interstitial object is one that exists inside of the subjective mind, but happens to be shared across a large number of subjects (potentially everyone). Or, to be more specific, a large number of subjects all happen to have each their own subjective opinion, but these opinions align closely enough that it can be treated as though it were an object that extends outside of the subjects.
ExampleAmerica. America is not a physical object (though the concept of it may be mapped to a physical area). America is similarly not really a subjective opinion; while there may be plenty of varying opinions about it, were I to decide that America is my cup of coffee, my use of the term would very quickly run into contradiction with pretty much everyone else. Instead, America is an idea shared by many different people, and it is because everyone roughly agrees on concepts like "where is America?" that things like 'borders' exist.
ExampleRacism. Racism is not a physical object (again, though the concept of it may be mapped to particular actions, and race may be ascribed to particular people). Racism is similarly not a subjective opinion; closing your eyes and saying "nope, it's not real" doesn't make it magically vanish. Instead, racism is a widely shared set of beliefs, and it is because everyone roughly agrees on expressions of culture and appearance that something like 'race' exists.
Both of the examples should make it painfully obvious that interstitial objects, despite being constructed on top of subjective reality, really lies somewhere between objective and subjective realities (in terms of its apparent permanence and tangibility).