The Jat-Krula, sometimes translated as the Maginot Sphere or the Line, is an array of Forerunner fortifications designed to protect the core worlds of the Ecumene, evidently located somewhere in or near the Orion Complex, from external attack. These fortifications include multiple layers of slipspace-interdiction weaponry, higher-dimensional disruptor fields, sensor networks and early-warning outposts.

The Jat-Krula is a complex structure, comprising millions of defensive stations spread across a bubble of thousands of light-years, all calibrated specifically to block any incoming approach vector from the outside galaxy. As such, its perimeter is hardly a straight line or even a bubble; the line installations' pattern is almost random, but has its own logic to it - one that is only apparent when viewed from within Slipstream space; the closest terrestrial analog might be an early modern star fort, but in three dimensions. Single line installations hardly protect any location in particular, but work in concert with the others to form a supposedly impregnable chain. This, however, also was the cause of the system's downfall. If even a handful of facilities were disabled, it would compromise the entire Line. By the present day, only a small fraction of the system remains active and in synchrony with the rest of the Jat-Krula array, but the relative few (which still include tens if not hundreds of thousands of stations) have still caused considerable inconvenience to the modern civilizations, especially the Covenant as they encroached on the Forerunner core worlds.