Beneath the Buy More is a high-end CIA facility that houses enough firepower to field a small army.
It has been occupied by many spies, including good guys and bad guys. Virtually every spy character on the show has been in there, and even some who either weren't spies or shouldn't have ever discovered the facility.
This base of operations for Team Bartowski, eventually became the headquarters for Carmichael Industries. It survived attempted and successful invasions, and even explosive destruction from above.
Plus, it was home to a naked Brit super spy and his hot make-out with Sarah. Plus hot sex between Captain Awesome and Ellie and I'm sure Chuck and Sarah explored each other's bodies in several different rooms and cells, unfortunately never to be seen by anyone, especially fans of the show.
Castle also had several forms of entry, depending on the episode. Sometimes it seemed like you needed to have some serious access to even get to a door that allowed you inside, while other times one could virtually happen in on the place by accident. Perhaps they had a dynamic security system that lowered and raised its level just to keep the team on its toes.
The place also seemed to change over time and even increase and decrease in size. At one point, there was a whole other HQ within Castle, where a competing team of Intersects was housed. It was also a place where the Bartowski clan all met at one point or another because of a mission or spy objective. Forever tying that place to the family profession.
Along with the Buy More, Castle has been a fixture (though, the Buy More has been around longer) and is a set piece that defines the show. It has delivered drama, comedy, romance, happiness, sadness, and loads of danger along the way. Without it, the show just wouldn't feel the same.
Woohoo! I'm almost done! Oh crap... that means the show is almost done.