The Arcane Sanctuary is a frozen castle sitting on a raised mounts of frozen crust on the planet Hecatia in Void Space. The structure itself is sealed shut from the outside and some unknown method blocks all attempts to scan the inside or penetrate the outer surface. Inside the structure reads as non-existant, with no energy or matter being read within. Since the inside reads as the absence of everything, transporters are unable to lock onto the interior and the frozen walls resist all attempts to breech them.
Interior
The interior may be accessed directly by portal, even within the main archway the gates are sealed and frozen shut. Upon entering the structure it is clear that they are no longer in a normal material realm that follows Euclidean geometry or normal physics. The inside of the structure folds back in upon itself and creates an endless loop of walkways that seem to float in an endless dark fog of cold, though the walkways themselves are warm enough to exist in without major environmental protections. The interior resides at a brisk 6 degrees Celsius (roughly 43 degrees Fahrenheit). These endless walkways are made of solid granite with a surface which is textured enough to provide traction even to bare feet but otherwise clearly ground down and milled to be flat. The granite walkways vary in size from just enough for a single person to comfortably walk all the way to several meters wide.
The walkways are all straight and exist at 90, 180, or 270 degree angles. Each walkway can very in length from just a meter or two all the way up to several kilometers long. The mist makes it difficult to tell distance between locations as locations quickly fade into the dark mist. The walkways have spires on the underside which appear to float in mid air without assistance in most cases. Other than walkways there are platforms, square or rectangular shaped areas that are both wide and long, stonework benches, beds, chairs, tables, and even fire-pits are spotted throughout usually between different walkways joined by stairs. At the end of each walkway are stairs that curve around in a spiral pattern changing height and direction to a new walkway. Since the space bends back into itself, the interior space is nearly infinite existing in some kind of pocket-dimension confined within the physical exterior of the castle itself. The exterior can be seen, however, on platforms there are floating glowing portals which view the exterior, it isn't clear and is one-way, those viewing the outside from the inside cannot be seen from the outside.
The stairs themselves are clearly designed for a humanoid with roughly human proportions to walk on and they float without assistance locked in place by some unseen force. They cannot be moved out of place through any known means.
Within the sanctuary there are a number of disorienting properties. For one outside of the walkways, platforms, and stairs gravity does not exist and this makes inertial guidance difficult at best. Further many of the walkways are identical and thus navigation by sight can also be a problem. The only way to clearly navigate is to have a clear image of where one wants to go and eventually one will get to that location.
Contents
The sanctuary itself could hold unlimited supplies. However they would need to be brought in from outside and some have been. Piles of dry wood are found near fire pits and often on a platform with surfaces to either prepare or eat food on, there are chests with food and water within, though it is unclear how they got to be there and often animals will look exotic and unfamiliar to those looking at them. Variations on known animals, coming from unknown places.
Travel
Platforms will also contain constructed portals which can lead to a variety of different places. These portal are usually to a subset of places on a certain world or plane. However, there is a central location where a large alter sits with a huge and ancient tome on a stand just in front of the raised circular alter. On this platform the edges have runes glowing in mid air made of magical energy which are changed with instructions in the tome using spell components and commands from the alter. Changing the runes alters the alignment of the sanctuary to other planes and can be used to open portals to literally anywhere, any times. Using the main portal will also change the alignments of the smaller portals throughout, though the same destinations are possible their location within the sanctuary will change.