Alternatives to map-and-key

[soviet]

I think there are rules at play in how I assess the fictional state. If the players say they want to sneak across the grounds, I set the DC at 10, and they roll high enough, then I can't decide that they have not made their way across the grounds.

The guard patrol changes shift, make another stealth roll.
The ground here is more gravelly, make another stealth roll.
You're near the inner palace now, make another stealth roll.
You're climbing now? Make another stealth roll.

Likewise if in their infiltration they take a clever set of actions and roll well, the rules of the game (& the world) dictate that they have achieved their result. I narrate that result, but I do not decide it.

There are no specific rules in D&D for 'infiltrating a compound'. There are separate rules for stealth, climbing, perception, etc that rely on the DM to put them together in what seems to them to be an appropriate way. That's a decision.

In the context of the overall challenge, I am never thinking "their goal is to bypass the outer defenses. Have they done that yet?" I think "what is the DC for stealth? Did they succeed? What is the climb DC? What are the consequences? Did they succeed?"

These are exactly the same thing.

I am adjudicating the world as an observer, not deciding as a participant.

The world doesn't exist to be observed. It's all made up. You are making a decision as to which parts of that made-up world you give weight to, and which additional fiction you do or do not introduce.