Defense seems always the most economic way when the defender is resourceful. At the same time offense seems a better strategy for attackers who have nothing to lose.
And when your capital city has a unique geography that makes it both difficult to attack and guaranteed to be a center of commerce.
I love all the giant old stone walls in Istanbul, the place is filthy with them. The people are totally used to it too - it's trippy to see schools and homes built around centuries-old ramparts.
Regrettably, that tactic is properly near impossible to use today - the US can't defend her own borders, and the world is so interconnected that stopping all foreign trade would result in mass starvation.
The wall doesn't need to be physical. It works as obstacles to slow down movement of the enemy.
For example, Google has built up its own wall by making practical extremely large computing platform. That's a big wall for them to deter competitors. Apple focus on user experience and that is a big wall for them to fend off copycats.
If U.S. has to build a big wall. The wall has to be something that make U.S. an aspiration for those enemies of our enemies, such as rebel students in Tehran or Beijing.
The U.S. has awesome walls: the Atlantic and Pacific oceans. They saved our butt numerous times in WW2, and arguably are the reason that we're a global superpower today.
Sure, but today all it takes is a few people determined to kill themself - with all the technology that is available today, the power of a small dedicated group is vastly bigger today that it was 65 years ago, and it is only likely that it will get even bigger.
