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?