Assyria base is located deep within a planet that is at the center of a field of asteroids, the star which used to be at the heart of the system went cold and died eons ago.
The asteroids have a unique property of containing Multidimensional Matter. The existence of which is thought to be only a theoretical mathematical construct by even the Vulcan Science Academy. The unique properties of the asteroid field push the planet just out of phase with normal space.
The special nature of the asteroid belt makes the planet itself and its surrounding space is obscured from sensors, existing in normal space but out of phase just enough to be invisible from the outside. The entire system moves through space in a roughly elliptical orbit around the galactic center with moderate deviations caused by nearby gravitational forces as it moves.
The base itself is an automated repair, resupply, and docking facility. Though it is capable of supporting a large crew compliment, the base was established by Section 31 and its systems were left entirely automated. A Section 31 space craft may can access by supplying a specialized code via subspace and then it receives instructions on a randomized coded channel. These instructions allow a space craft to enter the system from nearly anywhere in the galaxy using high energy warp field acting along the gravitational force of a star or other super-dense stellar body of at least .5 solar masses and above. The specifics of the warp field geometry, speed, and course change continuously making accidental entry or re-entry of a vessel without authorization nearly impossible.
The ability to instantaneously slingshot to Assyria Base is a unique property caused by the dense field of multidimensional matter. Exiting the system in a similar manner is also possible, once again, specific instructions are provided by the base computer system. Returning to the point of origin is far more difficult as well, making the act of leaving far more hazardous than entry. Several Section 31 vessels have been lost attempting this, through it is unclear as to exactly why such attempts failed, it is usually seen as some kind of pilot or operational error.