[Cass]

That's a bizarre line of thought to me. If you build an expensive structure for fortification, you don't usually get to the interior design and then go "Oh fuck it, this extra safety measure wouldn't cost anything, but if they've got this far we might as well surrender, so let's not bother."

Going by that logic, the president's bunker under the pentagon would've been built without a lock. After all, people don't usually have to physically drag a country's leader out of their locked bunker, right? By the time anyone's knocking on that door, usually the war is lost and the country has surrendered.

And yet, if you're designing for defense, why NOT take such a cheap and easy countermeasure as putting a lock on the door or choosing the more defensible way to spiral your staircase? You might want to buy a few more minutes to negotiate in a desperate situation; you might want at least the option of taking that futile last stand; you might be facing not an invading army but a single lunatic with a sword who snuck past the outer guards.