Principality is the realm of the Principle, both literally and figuratively. The realm is itself where the Principle resides, and is also literally the Principle itself. To be in Principality is to be within the Principle's absolute power, and within the Principle itself. In a sense, Principality is what you call the Principle when referring to it as a place.
Within Principality, the power of the Princes is nearly boundless. Though each Prince will be naturally more inclined to work along the lines of their personal portfolio, any Prince can create anything within Principality, so long what they create is not antithetical. Unlike in Natura, in Principality, Princes can work instantly and completely, translating thought into reality with zero effort in no time at all. Thus, I expect Principality to at least in part be an ever-shifting dreamscape, constantly changing to reflect the whims and designs of the Princes.
The Adversaries cannot enter, touch, threaten, perceive, or affect Principality in any way. They can, with difficulty and some effort, sense worldgates leading to Principality.
One thing that is NOT possible in Principality is the creation of sentient life. True sentience involves free will, which by its nature includes a freedom to explore antithetical thoughts and actions. Only the Princes are capable of harboring antithetical thoughts within Principality.
However, it is quite possible for a Prince to create a simulacrum or simulation of sentience within Principality. The creatures or spirits so created would not actually be sentient beings, but rather projections of the Prince's own mind. Should the Prince running a given simulation leave Principality or even simply stop paying attention to it, the simulation would cease to exist.
Time travel is also not possible in Principality. It is possible to run simulations at a faster than normal rate, and it is possible to alter someone's experience of time so that more or less time has passed according to their senses, but time will still have passed at a normal rate. In other words, you can't use the power of Principality to heal a Prince's wounds instantly, nor can you use Principality to travel back in time.
Living mortals cannot enter or exist in Principality; the raw purity and divinity of the place would rend their flesh and spirits. It is not even possible for a living mortal to pass through a worldgate; should they try, they would simply die, and their spirit would linger in Natura (unless it was already very principled). Only the mortal dead can exist in Principality, as they are much more static and don't have physical needs. And even then, in order to pass through a worldgate, they would need to be quite principled and purified.
Any materials, light, energy, or other stuff created within Principality cannot leave Principality. The nature of such stuff is too fundamentally different from Natura; the transition simply isn't possible.