The "Knight's Lair" is in reference to the base of operations used by Jason Knight, aka MidKnight X.
When MidKnight X first started with the Guardians of the Dawn superhero group, he spent most of his time operating from their base on Talos Island. But as he became more and more unhinged, he began to set up shop in Kings Row to stay close to his self-appointed domain.
In his home universe, Jason (then as Jason Rich) operated from the same base that his predecessor and legal guardian operated from in Hell's Kitchen. It was a modest space in an old apartment, but it put him in the middle of trouble and it allowed him to seemingly appear and disappear like magic.
Sadly, of course, this was destroyed along with the rest of his universe.
Not long after making Kings Row his "domain", the United Nations group Vanguard provided Jason with an apartment all his own and the offer of a civilian identity and a job to balance out his hero career. Jason accepted the apartment, but not the rest of the package.
One of the things he did was to sub-let the apartment, which gave him the extra money to get the supplies needed to make his "real home" happen.
Jason learned early on that the sewer system in Paragon City was almost a city in and of itself. All sorts of criminals made the sewers their home, giving them the ability to travel without detection and hide from the heroes. What better place to have his own lair?
Jason chose an abandoned water-treating platform in the sewers below Kings Row for his base of operations. He hollowed out the machinery and turned it into a livable facility, complete with a small corner to sleep for his one and only hour per day. The majority of the facility, though, was dedicated to his equipment, which allowed him to access cameras and communication signals all over Kings Row. It even had with his own first-generation CPCU, which he got cheap from a disbanded superhero group.
Eventually, however, as he became more obsessive, he spent more time underground than above it. It helped make him more of a recluse and also facilitated his eventual corruption into the apostle for Mot, not just once, but twice.
When Jason realized this, he knew he couldn't stay in the sewers. After his final release from Mot's influence, he triggered the facility's self-destruction, which overloaded the small reactor until it obliterated everything within a fifty-yard radius.
Following his "sleep", Jason knew that he needed to relocate his base of operations to someplace closer to Kings Row than in the sewers. With his new job as Director of Operations for Architect Entertainment, Jason realized that he had the best location in Kings Row right there.
He took an otherwise abandoned area of the AE Building dubbed "Level Thirteen" and turned that into the new Knight's Lair.
The new Lair contains far more computer equipment than before, some of which was "borrowed" from AE, and others were repurposed equipment from the Council and other criminal organizations.
Much like his sewer location, "Level Thirteen" has a sleeping area for him to get some rest and also to change outfits.
Sentry is Jason's personal CPCU system that is based in the Lair. Sentry actually was a failed attempt from the Council to adapt a Rikti computer system to their technology. They did not realize that the only thing that was wrong with their attempt was a defective power converter. Once Jason replaced that, he had his own CPCU system.
Sentry has an artificial intelligence along with a personality program that was previously used by Databank for a limited period of time. It has, at least to date, been effective in giving Sentry a more human personality to converse with.
Sentry is connected to the same networks as the Guardians of the Dawn, and also with several "backdoor hacks" to other networks such as the Paragon City Police Department.
The Sewer Lair was first seen in "Guardians of the Dawn GRADUATION" #2-3.
"Level Thirteen" was first seen in "Furia and the Guardians" #8. It was inspired by the "Bat-Bunker" under Wayne Towers in DC Comic's "Batman and Robin" series.