Ron Paul's Fortress America: How would it work?

People talk about Ron Paul but I can never get anyone to elaborate on the pros and cons of his Fortress America concept.

Build big walls. We hide behind them. We have a few missiles. Anyone who messes with us we annihilate.

That can't work well or at all.

Can anyone elaborate on what it would really be like and include the pros and cons of Ron Paul's sophomoric sounding concept?