a network of tunnels and sensors is easier to defend than a open space exposed position where drones can see you from miles away, I guess that makes tunnels better for defense

Given that we have bunker busters that will travel beyond the horizon at hypersonic speeds and bust 200 feet of reinforced concrete...

I don't think any fixed installation is particularly easy to defend?

Also, I'll grant that a drone can see you from miles away, but don't you think any one of a large and growing number of satellites can spot your massive earthworks from tens and hundreds of miles away for the months or years it takes you to construct?

Given that we have nuclear weapons why bother making anything?

The Russian invasion of Ukraine has been going the other way around entirely.

It's attackers that can't defend against anything.

they did - that's why dorne was never conquered

Little known fact, dragonglass is an excellent semiconductor

the operator of those would get hit by an aerial drone from one of the other tunnel exits

But now you've forced your adversary to deploy and use "this little guy" on top of everything else they're doing. If you raise the costs high enough that they can't pay, you win. Yay!