Fortify the Perimeter does exactly as the name implies; it fortifies the perimeter of your base by adding MG's to the top of each building, and significantly increases the health of all bunkers and observation posts. In addition, this upgrade allows units to reinforce near bunkers, up to a 25 unit radius. For only one CP, this advancement does more than enough to give Wehrmacht players the advantage in battle.
The decision to choose the Defensive Doctrine over the Terror Doctrine is typically due to this upgrade. Coming in cumulatively at two command points, this ability comes in early enough to make medical bunkers unbeatable without a massive expenditure of resources, or until armor arrives. Accordingly, units do not even need to retreat because they can be reinforced next to the medic bunker. The exact bonus for the medic bunker is an extra 300 HP, while all observation posts receive an extra 200 health, which is usually forgotten among most players. Finally, all base buildings receive MG's on the top, which negates any chance for a base rush during the game.