In the multiplayer or skirmish worlds of Command & Conquer (and most RTS's) you will notice many strategies, one of which is turtling. Turtling is when a player worries most about resource management and base defense rather than building up a huge army of poorly upgraded basic units. It is often the strategy that you see in long, dragged out games.
GDI
GDI has anti-armor oriented defenses, while units are good at taking care of enemy infantry. There still is a big hole in anti-air defense (though in KW it is taken care of by Slingshots and Tungsten Shells upgrade).
- Watchtower is GDI's basic anti-infantry defense. It's twin machine gun doesn't do much damage (in KW AP Ammo upgrade significantly increases the amount of damage) but it still works. It is recommended to build a Watchtower to your expansion to protect the expansion from enemy infantry. Watchtower has also stealh detection, which obviously helps to see enemy camouflaged units.
- Guardian Cannon is meant to protect from enemy vehicles, what it doesn't do very well (though in KW Railgun upgrade helps much). Still it works fine but it is expensive compared to its usefulness.
- AA Battery is GDI's base anti-air defense. Too bad that AA Battery without Tungsten Shells upgrade is rather useless. It doesn't do significant damage and most airships can attack out of AA Battery's range. The only reason to build AA Batteries in TW is to proctect base from Venoms or Stormriders.
- Sonic Emitter is GDI's advanced anti-armor defense. Its sonic waves cripples vehicles to pieces of metal and kills almost every man in its path. Compared to Nod Obelisk of Light and Scrin Storm Column in usefulness, Sonic Emitter in somewhere in between. Sonic Emitter is bit pricey, but a couple of Sonic Emitters with proper air defense is enough for even larger bases.
- Rig is one of the most underused units in the game and there are good reasons: it is too expensive, its armor doesn't take much hits and it's very vulnerable to infantry attacks. Otherwise Rig is versatile defense platform which has 2 Guardian Cannon turrets, missile anti-air defense, stealh detection and few repair drones. In KW Battlebases get Railgun upgrade which helps a bit but still they are useless
Nod
Nod's defenses are in good balance with each other and they are cost-effective. In KW there are several upgrades for defense turrets: Quad Turrets which adds 1 extra turret to Shredder, Laser and SAM Turrets, Charged