Defence structures in Art of War 3 can have a crucial role in the game depending on your strategy, they are effectively buildings that provide firepower. There are several factors to consider when building these structures. First: it takes up space, this means you will have less space for other buildings you might need. Second: it require energy to operate and their function can be taken away if Power Plants are destroyed to the point there isn't enough energy produced. Third: these buildings provide firepower without the cost in CP (Command Points).
Air Defense serves as a deterrence factor for enemy fighters and tactical bombers. Highly effective when combined with "Air-raid warning" Boost.
Bunker is basic Confederate Defence building, mainly effective against infantry, otherwise its job is to soak part of the damage.
When you need something that is dealing tons of damage to the Armored Vehicles in form of splash damage, build Rocket Tower.
Torpedo Platform purpose is twofold. On one hand, it protects against Naval forces. Its secondary role is to help in dealing with an enemy Aviation.
Build the Wall. When having a problem of dealing with unwanted guests within your base, consider erecting a wall in their path and prevent them from further advancement.