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?

Makes me think the noble houses of Westeros should have dug tunnels to foil the Targaryen dragon advantage.