I drove right by that place every single morning for a few years while working my last job. They kept the warning radar complex going near Concrete ND, which as part of it. Some pretty advancd phazed array radar for its day.
Another part of the system was the launch complexes for Spartan and Sprint missiles. The 1972 SALT 1 agreement limited the USA to two ABM sites. One was intended to be built to protect Washington, DC. The one actually built was to protect the GFAFB Minuteman wing.
The complex was declared operational on 1 October 1975. The next day Congress ordered the site deactivated.