Notice that he specified "fixed" fortifications, which given the time period were things like the French Maginot Line or the German Atlantic Sea Wall.
The bottom line is that there is no "impenetrable fortress".
That doesn't mean that fortifications are stupid. What is stupid is to be so invested in a piece of terrain as to give up the ability to maneuver trying to hold it. The Russians have twice used a campaign of defensive action to good effect, first against Napoleon and second against the Third Reich.
The first problem with fixed fortifications is that they give the enemy an exact location of where you will be, and most likely when you will be there. This means that the enemy chooses the time for the battle even if you chose the place.
The second problem is that a defensive posture is inherently passive. How many men do you have guarding the others for the guard rotation? An old trick is to send small probing parties to bring a fortification to 100% alert for two or three days in a row, denying the defending force sleep while the attackers rest up for a massed assault.
In Vietnam the idea of mutually covering firebases took hold, and still the enemy was able to overrun several due to environmental factors such as weather making the use of air support and artillery impossible. The Soviets in Afghanistan (and the US Military in Iraq) congregated on large bases (GEN Petraeus used the troop surge to push combat power off of FOB's and into neighborhoods with good effect).
Infantry doctrine calls for an "offensively minded defense" meaning that the goal of the defense is still to close with and kill the enemy. Such an example would be to have an Engineer squad prepare dug in fighting positions along a route of egress so that the covering forces can slow the progress of a pursuing force. The Prussian "discipline of the spade" followed the Roman example and made the infantry dig in whenever stopped. A sound tactic for staying alive.
Now how do we apply these lessons to the American Militia? Well first don't get cornered in compounds or houses. The inevitable result is Ruby Ridge or Waco. To be effective you have to maintain the ability to maneuver, even if you don't have freedom of maneuver. After all the city fighting in Stalingrad saw entire regiments moved one man at a time through holes and tunnels single file.
To sum it up, always have an escape route. Better yet, have two or three. Because if you make it REALLY hard for the enemy to get in, you make it really hard for yourself to get out.