be like Fort Knox

To be inaccessible, usually because the item or place in question is locked or guarded. Fort Knox is a military site in Kentucky where stores of gold are kept. The kids are home by themselves, but don't worry, the house is like Fort Knox with all the security cameras. I can't get into the safe, it's like Fort Knox! We'll never be able to sneak in, this building is like Fort Knox after hours.

be like/as safe as Fort ˈKnox

(about a building) be strongly built, with many locks, strong doors, alarms, etc. so that it is very safe and difficult for thieves to enter: This home of yours is like Fort Knox. ♢ Financially she’s as safe as Fort Knox. Fort Knox is a military base in Kentucky where most of the US’s store of gold is kept.